Vegan Mushroom Oat Burgers with Caramelized Onions
Meaty, deeply savory pan-fried patties made from browned cremini mushrooms, rolled oats and white miso, piled on toasted buns with sweet caramelized onions, peppery arugula and a quick mustard mayo.
Steps
Before you start
- Plan on about 2 hours total; the onions take 35 minutes and the patties need 20 minutes in the fridge to firm up.
- Use old-fashioned rolled oats, not instant or steel-cut; instant oats turn gluey and steel-cut oats stay hard.
- Measure and prep everything first: slice the onions, mince the garlic and chop the mushrooms before any pan goes on the heat.
- Check that your buns and mayonnaise are vegan if you need a fully vegan meal.
Caramelize the onions
- Heat the onion olive oil in a 12-inch (30 cm) skillet over medium-low heat for 1 minute.
- Add the sliced onions and ¼ teaspoon salt and stir to coat.
- Cook for 30–35 minutes, stirring every 3–4 minutes at first and more often near the end, until the onions are soft, deep golden brown and jammy.
- If they start to stick or scorch, add 1 tablespoon of water and scrape the pan.
- Stir in the balsamic vinegar for the last minute, then scrape into a bowl.
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Ready when: Onions are a deep amber-brown, very soft and sweet-smelling, reduced to about one-quarter of their raw volume
Tip: Don't rush with higher heat; browned-too-fast onions turn bitter rather than sweet.
Make the flax gel
- In a small bowl, stir together the ground flaxseed and 3 tablespoons cold water.
- Set aside; it will thicken into a gel while you cook the mushrooms and serve as the binder.
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Ready when: Mixture looks thick and slightly stringy, like beaten egg white
Chop the mushrooms
- Working in 2–3 batches, pulse the quartered mushrooms in a food processor with 6–8 one-second pulses until finely chopped into pieces about ⅛–¼ inch (3–6 mm); don't let them become a paste.
- (By hand, chop finely with a sharp knife.) Pulse the rolled oats 3–4 times in the same processor and set aside.
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Ready when: Mushroom pieces look like coarse crumbs with no large chunks; oats are about half broken up
Tip: Overprocessed mushrooms release too much water and make soggy patties.
Brown the mushrooms
- Heat the mushroom olive oil in a second large skillet over medium-high heat until shimmering.
- Add the chopped mushrooms in an even layer and cook, stirring every minute or so, for 10–12 minutes.
- They will release liquid, then the liquid will cook off.
- Add the garlic, thyme and smoked paprika in the last minute and stir until fragrant.
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Ready when: The pan is dry (no liquid pooling when you tilt it) and the mushrooms are browned and noticeably shrunk
Tip: Don't stop early; leftover moisture is the main reason veggie burgers fall apart.
Season with miso
- Stir the miso and soy sauce together in a small cup until smooth.
- Add to the mushrooms and stir over medium-high heat for 1 minute so the mixture is evenly coated and glossy.
- Remove from the heat.
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Ready when: Mushrooms look dark, glossy and slightly sticky with no visible lumps of miso
Cool the filling
- Spread the mushroom mixture on a plate or in a wide bowl and let it cool for 10 minutes, until warm rather than hot.
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Ready when: Mixture is just warm to the touch
Mix the patty mixture
- In a large bowl, combine the cooled mushrooms, pulsed oats, flax gel, black pepper and one-third of the caramelized onions (about 3 tablespoons; chop them roughly first).
- Mix with your hands for about 1 minute, squeezing, until the mixture holds together when pressed in your fist.
- Reserve the remaining onions for topping.
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Ready when: A handful squeezed together holds its shape without crumbling; if too wet, add 1 tablespoon more oats
Tip: If it feels too dry and crumbly, add 1 teaspoon water at a time.
Shape and chill
- Divide the mixture into 4 equal portions (about 150 g each).
- Press firmly into patties about 4 inches (10 cm) wide and ¾ inch (2 cm) thick, pressing a shallow dimple in the center of each.
- Place on a parchment-lined baking sheet and refrigerate for 20 minutes.
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Ready when: Patties feel firm and hold their shape when gently lifted
Tip: Don't skip the chill; unchilled patties are fragile and break when flipped.
Pan-fry the patties
- Heat the frying oil in a 12-inch (30 cm) nonstick or cast-iron skillet over medium heat until shimmering.
- Carefully add the patties.
- Cook for 4–5 minutes without moving, until the underside is deep brown and crisp.
- Flip gently with a wide spatula and cook another 4 minutes.
- Transfer to a plate.
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Ready when: Both sides are dark golden-brown and crisp, and the patties are hot all the way through (steaming when pressed)
Tip: Flip only once, and wait until the crust releases easily from the pan.
Toast buns and assemble
- Stir the vegan mayo and Dijon together.
- Toast the split buns cut-side down in the pan the patties cooked in (wipe out any burnt bits first) over medium heat for 1–2 minutes until golden.
- Spread the mustard mayo on both halves, then layer the bottom bun with arugula, tomato (if using), a patty, a generous pile of the remaining caramelized onions, and the top bun.
- Serve immediately.
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Ready when: Buns are golden and lightly crisp on the cut sides
Tip: Put the arugula under the patty so the heat doesn't wilt it entirely.
Good to know
Tips
- Cook the mushrooms until the pan is completely dry and they begin to brown; that evaporation concentrates flavor and prevents crumbly, soggy patties.
- Handle the patties gently once they are in the skillet, and use a wide spatula for flipping.
- For extra crunch, press a thin coat of dry oats onto each patty before frying.
- Taste the mixture before shaping: if it needs more salt, add a pinch rather than more soy sauce or miso, which can overpower.
Make ahead
Caramelize the onions up to 4 days ahead and refrigerate. Shape the patties (step 8) up to 1 day ahead, cover and refrigerate on the parchment-lined sheet, then pan-fry straight from the fridge, adding 1 minute per side if needed.
Storage & reheating
Cooked patties keep in an airtight container in the fridge for up to 4 days. Freeze uncooked or cooked patties, separated by parchment, for up to 3 months. Refrigerate leftover caramelized onions for up to 5 days. Room temperature: no longer than 2 hours. Reheat patties in a lightly oiled skillet over medium heat for 3 minutes per side, until hot in the center (from frozen, thaw overnight in the fridge first).
Substitutions
- white miso paste → Red or yellow miso, or 1 extra tablespoon of soy sauce plus 1 teaspoon tomato paste. Red miso is saltier and stronger, so use 2 teaspoons; the soy-tomato version is less rounded but still savory.
- ground flaxseed → 1 tablespoon chia seeds ground, in the same amount of water. Works equally well as a binder; let it sit 5 minutes to gel.
- old-fashioned rolled oats → Certified gluten-free rolled oats and a gluten-free bun. Regular oats may be cross-contaminated; patties are otherwise the same. Soy sauce can be swapped for tamari.
- cremini (baby bella) mushrooms → Shiitake caps or a mix of white button and portobello mushrooms. Shiitake gives a stronger, meatier flavor; remove tough shiitake stems. Portobello gills can darken the patties.
