Slow Cooker Lentil Soup

Name: Slow Cooker Lentil Soup
Prep time: 25 min
Cook time: 7 h 45 min
Total time: 8 h 10 min
Yield: 6 servings
Cuisine: American
Ingredients: 2 tablespoons (30 ml) extra-virgin olive oil; 1 large (about 9 oz) (250 g) yellow onion, peeled and cut into ½-inch (1 cm) dice; 3 medium (about 8 oz) (240 g) medium carrots, peeled and cut into ½-inch (1 cm) dice; 3 stalks (about 5 oz) (150 g) celery stalks, cut into ½-inch (1 cm) slices; 4 cloves (12 g) garlic cloves, minced; 2 tablespoons (32 g) tomato paste; 1½ teaspoons (3 g) ground cumin; 1 teaspoon (2½ g) smoked paprika; 1 teaspoon (1 g) dried thyme; 1½ cups (300 g) dried brown lentils, rinsed well and drained; 14½ oz can (411 g) canned diced tomatoes, undrained; 6 cups (1420 ml) low-sodium vegetable broth; 2 leaves (1 g) dried bay leaves; 1 teaspoon, plus more to taste (5 g) kosher salt; ½ teaspoon (1 g) black pepper, freshly ground; 5 oz (about 5 packed cups) (140 g) baby spinach, roughly chopped if the leaves are large; 2 tablespoons (30 ml) fresh lemon juice, from about 1 lemon

From the MyKitchenBank Kitchen

A hearty, set-and-forget vegetarian soup of brown lentils, carrots, celery and tomatoes, built on a quick sauté of onion, garlic and tomato paste for deep flavor. Spinach and a squeeze of lemon at the end keep it bright.

Prep: 25 min
Cook: 7 hr 45 min
Total: 8 hr 10 min
Makes: About 9 cups (6 generous servings)

Jump to recipe ♡ Save

Ingredients

6 servings

2 tablespoons (30 ml) extra-virgin olive oil
1 large (about 9 oz) (250 g) yellow onion, peeled and cut into ½-inch (1 cm) dice
3 medium (about 8 oz) (240 g) medium carrots, peeled and cut into ½-inch (1 cm) dice
3 stalks (about 5 oz) (150 g) celery stalks, cut into ½-inch (1 cm) slices
4 cloves (12 g) garlic cloves, minced
2 tablespoons (32 g) tomato paste
1½ teaspoons (3 g) ground cumin
1 teaspoon (2.5 g) smoked paprika
1 teaspoon (1 g) dried thyme
1½ cups (300 g) dried brown lentils, rinsed well and drained
14½ oz can (411 g) canned diced tomatoes, undrained
6 cups (1420 ml) low-sodium vegetable broth
2 leaves (1 g) dried bay leaves
1 teaspoon, plus more to taste (5 g) kosher salt
½ teaspoon (1 g) black pepper, freshly ground
5 oz (about 5 packed cups) (140 g) baby spinach, roughly chopped if the leaves are large
2 tablespoons (30 ml) fresh lemon juice, from about 1 lemon

Equipment (7)

5- to 6-quart (4.7–5.7 L) slow cooker
12-inch (30 cm) skillet or large sauté pan
Chef's knife and cutting board
Fine-mesh sieve
Wooden spoon
Ladle
Potato masher or immersion blender (optional, for thickening)

Steps

Before you start

Plan on 7 to 8 hours on LOW (or 4 to 5 hours on HIGH) of slow cooking, plus about 25 minutes of hands-on time.
Use brown or green lentils. Red lentils break down into mush over this long a cook, and they will not give you the same texture.
Rinse the lentils in a fine-mesh sieve and pick out any small stones or shriveled lentils.
This recipe is sized for a 5- to 6-quart (4.7 to 5.7 L) slow cooker. Do not fill the cooker more than about three-quarters full.

1
Prep the vegetables
- Rinse the lentils in a fine-mesh sieve under cold running water for 30 seconds, picking out any debris, then set aside to drain.
- Dice the onion and carrots into ½-inch (1 cm) pieces, slice the celery ½ inch (1 cm) thick, and mince the garlic.
- Keeping the pieces a similar size helps them cook evenly.
+ Show details− Hide details
Ready when: Vegetables are in uniform ½-inch pieces and the lentils are drained.
Tip: Smaller dice than ½ inch will turn very soft after 8 hours.
2
Sauté the aromatics
- Heat the olive oil in a 12-inch (30 cm) skillet over medium heat until it shimmers, about 1 minute.
- Add the onion, carrots and celery with the ½ teaspoon of the salt (use 1/2 of the 1 teaspoon total).
- Cook, stirring every minute or so, for 7 to 8 minutes.
+ Show details− Hide details
Ready when: The onion is translucent and soft, and the edges of the carrots have just begun to look glossy.
Tip: Don't let the vegetables brown; you want them sweet and soft, not toasted.
3
Bloom spices and paste
- Push the vegetables to the sides of the skillet.
- Add the garlic, tomato paste, cumin, smoked paprika and thyme to the center.
- Stir the paste and spices constantly for 1 to 2 minutes, then stir everything together.
+ Show details− Hide details
Ready when: The tomato paste darkens to a brick red and the mixture smells fragrant and toasty.
Tip: If the garlic starts to brown or the pan looks dry and sticks, lower the heat and add 1 tablespoon of broth.
4
Load the slow cooker
- Scrape the sautéed vegetable mixture into a 5- to 6-quart (4.7–5.7 L) slow cooker.
- Add the rinsed lentils, the undrained diced tomatoes, the vegetable broth, bay leaves, the remaining ½ teaspoon salt and the black pepper.
- Stir well, making sure all the lentils are submerged.
+ Show details− Hide details
Ready when: All of the lentils are covered by liquid.
Tip: Hold the salt at 1 teaspoon total for now. Broths vary a lot in saltiness, so you will season again at the end.
5
Slow cook the soup
- Cover and cook on LOW for 7 to 8 hours or on HIGH for 4 to 5 hours.
- Do not lift the lid during the first few hours, because each peek adds 15 to 20 minutes to the cooking time.
- Start checking at the low end of the time range.
+ Show details− Hide details
Ready when: Lentils are completely tender with no chalky or firm center when you bite one, and the carrots are soft when pierced with a fork.
Tip: Canned tomatoes are acidic and can slow lentil softening. If the lentils are still firm, cook another 30 to 60 minutes.
6
Remove bay and thicken
- Fish out and discard the bay leaves.
- For a thicker, creamier body, mash the soup 5 to 6 times with a potato masher right in the pot, or pulse an immersion blender 3 to 4 times, leaving most of the lentils whole.
+ Show details− Hide details
Ready when: The broth looks slightly thicker and more stew-like, with plenty of whole lentils still visible.
Tip: Mash only briefly. Over-blending gives a muddy, paste-like soup.
7
Wilt the spinach
- Stir the spinach into the hot soup in two or three handfuls.
- Cover and cook on HIGH for 5 minutes (or let it sit covered with the cooker set to WARM for 10 minutes).
+ Show details− Hide details
Ready when: The spinach is dark green, completely wilted and tender.
Tip: Don't add the spinach earlier, or it turns drab and slimy.
8
Brighten and season
- Stir in the lemon juice.
- Taste the soup and add more kosher salt, ¼ teaspoon at a time, until the flavors taste bold and balanced.
- Add more pepper if you like.
- If the soup is thicker than you like, stir in ¼ to ½ cup (60 to 120 ml) hot water or broth.
+ Show details− Hide details
Ready when: The soup tastes savory and bright, not flat, with a gentle tang from the lemon.
Tip: Lentil soup often needs more salt than you expect. Season at the end, then taste again.
9
Serve
- Ladle the soup into warm bowls, about 1½ cups (360 ml) each.
- Serve hot with crusty bread, a drizzle of olive oil, or a sprinkle of black pepper.
+ Show details− Hide details
Ready when: Soup is steaming hot throughout.

Good to know

Tips

Sautéing the vegetables and toasting the tomato paste first adds a depth of flavor that a straight dump-and-go version lacks. If you are short on time, you can skip it and add the raw vegetables, spices and paste directly to the cooker, but the soup will taste a little flatter.
Add the acid (lemon juice) at the end. Adding it early can leave lentils slightly firm.
A parmesan rind added with the broth (and removed with the bay leaves) gives extra savory depth, though the soup is then no longer vegan.
Slow cookers run at different temperatures. The first time you make this, check the lentils 1 hour before the minimum time.

Make ahead

"Chop the onion, carrots and celery up to 2 days ahead and store them in an airtight container in the fridge. Or assemble the sautéed vegetables, lentils, tomatoes and broth in the slow cooker insert the night before, refrigerate, and start cooking in the morning, adding 30 to 60 minutes to the cooking time because the insert will be cold."

Storage & reheating

"Room temperature: no more than 2 hours. Fridge: cool and store in airtight containers for up to 5 days. Freezer: freeze in portions for up to 3 months (leave 1 inch of headspace). Reheat in a saucepan over medium-low heat, stirring often, until steaming hot. Thin with a splash of water or broth, as the soup thickens as it sits. Thaw frozen soup overnight in the fridge first."

Substitutions

dried brown lentils → green or French green lentils (du Puy). Same amount and cooking time; French lentils hold their shape especially well. Do not use red or yellow lentils.
baby spinach → chopped kale (about 4 cups, stems removed). Add it with the spinach step but cook for 15 to 20 minutes on HIGH until tender.
low-sodium vegetable broth → chicken broth. Use an equal amount for a non-vegetarian version, or use water plus 2 teaspoons of bouillon paste.
fresh lemon juice → red wine vinegar or apple cider vinegar. Use 1 tablespoon and add more to taste.