French Baked Pasta Gratin with Leeks and Béchamel
A classic French-style baked pasta gratin featuring tender penne layered with creamy béchamel, caramelized leeks, and melted Gruyère cheese. The dish is finished under the broiler until the top is golden and bubbling. Elegant enough for company but straightforward to assemble, this is comfort food elevated with simple, quality ingredients.
Steps
Before you start
- Preheat the oven to 375°F (190°C)
- Have all ingredients measured and ready before you begin
- Use a 9×13-inch (23×33 cm) baking dish or similar 2-quart capacity vessel
- Clean the leeks thoroughly by slicing lengthwise and rinsing between layers to remove any soil
Cook the pasta
- Bring a large pot of salted water to a rolling boil.
- Add the penne and cook for 2 minutes less than the package instructions—the pasta should be just shy of fully tender since it will finish cooking in the oven.
- Drain in a colander but do not rinse.
- Set aside.
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Ready when: Pasta is pliable and yields slightly to a bite, with a firm core remaining
Tip: Cooking pasta slightly al dente prevents it from turning mushy in the oven.
Make the béchamel
- Melt 3 tbsp (42 g) butter in a medium saucepan over medium heat.
- Once foaming, add the flour and whisk constantly for 1–2 minutes to form a smooth paste (roux).
- Gradually add the warmed milk in three additions, whisking vigorously after each to incorporate without lumps.
- Once all milk is added, continue whisking occasionally until the sauce thickens, about 3–4 minutes.
- It should coat the back of a spoon lightly.
- Season with ¼ tsp nutmeg, salt, and white pepper to taste.
- Fold in ¾ cup (85 g) grated Gruyère until fully melted and smooth.
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Ready when: Sauce coats the back of a spoon and shows no lumps when stirred
Tip: Warming the milk before adding prevents the roux from seizing and makes a smoother sauce. Whisk constantly to avoid lumps.
Sauté the leeks
- Heat 2 tbsp (28 g) butter in a large skillet over medium heat.
- Once foaming, add the prepared leek pieces and the thyme sprigs.
- Cook, stirring occasionally, for 15–18 minutes until the leeks are very tender and the cut edges turn golden and lightly caramelized.
- The leeks should be soft enough to break apart with gentle pressure.
- Remove the thyme sprigs and discard.
- Taste and adjust salt if needed.
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Ready when: Leeks are very soft and pale golden at the edges, and a piece breaks apart easily under a wooden spoon
Tip: Do not rush the leeks—slow cooking develops their natural sweetness. Stir occasionally to ensure even caramelization.
Assemble the gratinOven 375°F / 190°C
- Preheat the oven to 375°F (190°C) if not already done.
- Spread one-third of the béchamel in the bottom of a 9×13-inch (23×33 cm) baking dish.
- Layer half the cooked penne over the sauce, then half the sautéed leeks.
- Spread half of the remaining béchamel over the leeks.
- Add the remaining penne, then the remaining leeks, and finish with the last of the béchamel, spreading it evenly to cover the top.
- Combine the remaining ½ cup (55 g) grated Gruyère with the Parmigiano-Reggiano and sprinkle evenly over the top.
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Ready when: All layers are visible and the cheese topping is distributed evenly
Tip: Spreading the sauce in layers rather than mixing ensures a creamy, stable gratin that doesn't dry out. Reserve the cheese topping for the final step.
Bake the gratinOven 375°F / 190°C
- Transfer the baking dish to the preheated 375°F (190°C) oven and bake for 25–30 minutes until the sides are bubbling gently and the top is pale golden.
- The sauce should show some movement at the edges.
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Ready when: Sauce bubbles at the edges and the top is pale golden
Tip: Baking at a moderate temperature allows the layers to heat through without the top browning too quickly.
Finish under the broiler
- Switch the oven to broil on high.
- Leave the gratin in the oven for 3–5 minutes, watching closely, until the cheese topping turns deep golden brown with occasional dark spots.
- The goal is a rich, burnished crust—do not walk away during this step, as broilers vary in intensity and the top can blacken quickly.
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Ready when: Cheese is deep golden brown with dark spots; avoid blackening
Tip: Move the baking dish to the highest oven rack before broiling if it is too close to the heating element. Stay in the kitchen and check every 30 seconds.
Rest and serve
- Remove the gratin from the oven and let it rest for 5 minutes before serving.
- This allows the layers to set slightly, making them less likely to slide apart when plated.
- Spoon onto warm plates and serve immediately.
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Ready when: Top is firm to the touch and no longer steaming heavily
Tip: Resting prevents the creamy interior from running across the plate and keeps the layers intact.
Good to know
Tips
- To avoid lumps in the béchamel, whisk constantly while adding milk, and ensure the roux is fully cooked before adding any liquid.
- If the béchamel seems too thick before adding the cheese, whisk in a bit more warm milk to reach a pourable consistency.
- The leeks can be cooked up to 1 day ahead and stored in an airtight container in the refrigerator. Reheat gently before assembling.
- For a lighter dish, replace half the whole milk with vegetable broth, but reduce the cheese in the béchamel to 6 tablespoons (70 g) to maintain proper consistency.
Make ahead
Assemble the gratin up to the cheese topping up to 4 hours ahead. Cover tightly with plastic wrap and refrigerate. Add an extra 10–12 minutes to the baking time if baking from cold. Do not freeze before baking, as the pasta and leeks can become watery upon thawing.
Storage & reheating
Refrigerate leftovers in an airtight container for up to 3 days. Reheat gently in a 350°F (175°C) oven for 15–20 minutes, covered with foil to prevent the top from drying out. Do not microwave, as the sauce can separate. The gratin does not freeze well due to the cream sauce breaking upon thaw.
Substitutions
- Gruyère cheese, finely grated → Emmental or Comté cheese. Both have similar melting properties and nutty flavor. Use 1¼ cups (140 g) total, divided as directed.
- fresh thyme sprigs → 1 tsp dried thyme. Use about one-third the amount of dried herb. Add to the leeks with the butter and omit the step of removing sprigs.
- penne pasta → Rigatoni or fusilli. Any short, sturdy pasta that holds sauce well will work. Avoid thin pasta like spaghetti.
- whole milk, warmed → Half-and-half or light cream. This will produce a richer sauce. Use the same volume (2 cups / 480 ml).
