Cauliflower and leek gratin pairs tender vegetables with a creamy, gently cheesy sauce and a crisp browned topping. The leeks bring a mild sweetness, while briefly cooking the cauliflower first helps it bake evenly without turning watery or mushy.
Ingredients
- 1 large cauliflower, about 2 pounds (900 g), trimmed and cut into bite-size florets
- 2 medium leeks, white and pale green parts only, thinly sliced
- 3 tablespoons unsalted butter, divided
- 2 tablespoons all-purpose flour
- 1½ cups (360 ml) whole milk, warmed
- ½ cup (120 ml) heavy cream
- 1 teaspoon Dijon mustard
- ¼ teaspoon freshly grated nutmeg
- ¾ teaspoon fine salt, plus salt for the cooking water
- ¼ teaspoon black pepper
- 1 cup (100 g) grated sharp cheddar, divided
- ½ cup (50 g) finely grated Parmesan, divided
- ½ cup (50 g) panko breadcrumbs
- 1 tablespoon chopped fresh thyme leaves, or 1 teaspoon dried thyme
Instructions
- Heat the oven and prepare the dish. Set a rack in the middle of the oven and heat it to 400°F (200°C). Lightly butter a 2-quart (about 2-liter) baking dish. Put a large pot of salted water on to boil.
- Par-cook the cauliflower. Add the florets to the boiling water and cook for 4–5 minutes, until a knife can just pierce a thicker piece but the cauliflower still holds its shape. Drain thoroughly in a colander. Let it steam-dry for a few minutes; excess water can thin the sauce.
- Soften the leeks. Melt 1 tablespoon of butter in a skillet over medium-low heat. Add the sliced leeks and a small pinch of salt. Cook for 7–9 minutes, stirring often, until soft and sweet but not browned. If they begin to catch, lower the heat and add a teaspoon of water.
- Make the sauce. In a saucepan, melt 2 tablespoons of butter over medium heat. Whisk in the flour and cook for 1 minute, stirring, without letting it brown. Gradually whisk in the warm milk, then the cream. Bring the mixture to a gentle simmer, whisking frequently, and cook for 3–5 minutes until smooth and thick enough to coat a spoon.
- Season and add cheese. Take the pan off the heat. Stir in the Dijon, nutmeg, ½ teaspoon of the salt, pepper, ¾ cup of the cheddar and ¼ cup of the Parmesan. Taste carefully and add the remaining salt if needed. The sauce should be savory but not overly salty, since the cheeses add salt.
- Assemble the gratin. Arrange the drained cauliflower and softened leeks in the prepared dish. Pour the sauce evenly over them and gently turn the vegetables so the sauce reaches the gaps. Avoid pressing them down; keeping some space helps the heat circulate.
- Add the topping and bake. Mix the panko with the remaining ¼ cup cheddar, remaining ¼ cup Parmesan and thyme. Scatter it evenly over the vegetables. Bake uncovered for 25–30 minutes, until bubbling around the edges and golden on top. If the top browns too quickly, loosely cover it with foil for the final part of baking.
- Rest before serving. Let the dish stand for 10 minutes so the sauce settles and portions hold together. Serve hot. For food safety, if checking with a thermometer, the center should reach at least 165°F (74°C).
Tips
- Clean leeks well. Sand can hide between the layers. Slice them first, swish the pieces in a bowl of cold water, then lift them out and drain; leave grit behind in the bowl.
- Drain thoroughly. After blanching, give the cauliflower time to steam-dry. Wet florets release water in the oven and can make the gratin loose.
- Keep the sauce smooth. Warm milk and gradual additions make whisking easier. If small lumps appear, whisk vigorously or pass the sauce through a fine sieve before adding the cheese.
- Choose your crumbs thoughtfully. Panko gives a light, crisp top; ordinary breadcrumbs work too but brown more quickly. Check early if using fine crumbs.
Serving Suggestions
Serve the gratin as a generous side with roast chicken, pork, baked fish or a simple green salad. It also works as a vegetarian main alongside crusty bread and a sharp, lightly dressed salad. Because the sauce and cheese are rich, fresh or acidic sides—such as lemony greens, pickled onions or sliced tomatoes—make a particularly good balance.
Variations
Swap the cheddar for Gruyère, Comté or mature white cheddar, keeping the total amount of cheese the same. For a stronger mustard note, increase the Dijon to 1½ teaspoons. Add 150 g (about 5 ounces) sliced, sautéed mushrooms with the leeks, or stir in a handful of chopped parsley after the sauce is made. For a gluten-free version, use a measure-for-measure gluten-free flour blend and certified gluten-free breadcrumbs; check that the blend is suitable for making a roux. For a less rich dish, replace the cream with more whole milk, though the sauce will be a little lighter.
Storage and Reheating
Cool leftovers promptly, ideally within 2 hours, then cover and refrigerate in a shallow airtight container for up to 3–4 days. Reheat individual portions in the microwave until steaming hot throughout, stirring or turning if needed. To warm a larger amount and revive the topping, cover the baking dish loosely with foil and heat at 350°F (175°C) for about 20–25 minutes; remove the foil for the final 5 minutes. Reheat only the portion you plan to eat, and make sure the center reaches 165°F (74°C). Freezing is possible for up to 2 months, but the creamy sauce may separate and the cauliflower soften after thawing; thaw overnight in the refrigerator and reheat thoroughly.
Frequently Asked Questions
Can I prepare cauliflower and leek gratin ahead?
Yes. Cook and drain the cauliflower, soften the leeks, and make the sauce up to a day ahead. Store each component separately in the refrigerator, then assemble with the freshly prepared crumb topping shortly before baking. If assembling the whole dish ahead, keep it covered and chilled, and allow extra baking time because it will start cold.
Can I use frozen cauliflower?
Yes, but thaw it completely and drain it very well first. Pat the florets dry and skip the boiling step, since frozen cauliflower is usually already blanched. The texture may be softer than with fresh cauliflower, so handle it gently when coating it with sauce.
Why is my gratin watery?
The most common causes are cauliflower that was not drained or dried enough, or a sauce that was too thin before baking. Drain the florets thoroughly and simmer the sauce until it coats a spoon. Avoid covering the gratin during most of baking, because trapped steam can also soften the topping.
Can I make this without cream?
Yes. Replace the ½ cup cream with ½ cup whole milk. The result will be slightly less rich but should still be creamy because the flour-thickened sauce and cheese provide body.
How do I know when it is done?
The edges should be bubbling, the topping golden, and a knife should slide easily into the cauliflower. If you use a food thermometer, check the center for at least 165°F (74°C), especially when reheating or baking the gratin from chilled.
Can I freeze the baked gratin?
You can, though the vegetables may become softer and the sauce less smooth. Cool it promptly, wrap it tightly, and freeze for up to 2 months. Thaw in the refrigerator before reheating, and heat until the center is steaming hot.