Roasting the leeks and cauliflower first makes them tender and gives them a crisp-around-the edges texture, imparting flavor to the finished dish.
*Ingredients can be purchased at the market.
Cauliflower*, about 3 pounds untrimmed (one large head or two small)
4 tablespoons olive oil
3 cloves garlic*, minced
1 large leek*
1 1/2 tablespoons butter
1 1/2 tablespoons flour
2 cups milk*
1/2 cup grated hard cheese*
1 teaspoon whole-grain mustard
Salt & pepper
1/4 cup breadcrumbs (purchased or homemade with market bread*)
Preheat the oven to 400°. Trim leaves and base of stem on cauliflower. Cut the cauliflower into florets that are 1 1/2 inches wide. You can chop the tender stalks up into pieces as well in this dish; everything will be covered with a creamy sauce.
In a large bowl, toss the cauliflower with 3 tablespoons olive oil. Season with salt and pepper, and spread on a large rimmed baking sheet. Roast until tender with just a bit of bite and browned, 35-40 minutes, stirring occasionally. Stir in the garlic after 20 minutes. (Be careful not to overcook once you add the garlic, or it will burn.)
Meanwhile, remove dark tops of the leek and cut lengthwise. Wash thoroughly (look for dirt in between the leaves at the top of the leek), then cut each half into 2-inch lengths.
Separate the layers of the leek and toss with the remaining 1 tablespoon oil and some salt, then spread out on another large rimmed baking sheet and roast until crispy in some parts and thoroughly soft throughout, about 15 minutes. (You can roast the vegetables several hours ahead and refrigerate before completing the dish.)
To make the cream sauce, use a medium saucepan to melt the butter over medium-low heat. Whisk in the flour and cook, stirring constantly, for 3-5 minutes. Do not burn or brown. Whisk in the milk until smooth, then bring to a simmer. Maintain the heat so mixture is bubbling a bit; stir often until the sauce thickens and the flour cooks, about 15 minutes.
Add 1/3 cup of the cheese and stir until just melted. Remove from the heat and season well with salt and pepper, then stir in the mustard.
When the vegetables are done, reduce the oven temperature to 350°.
Combine the roasted vegetables with the sauce and pour all into a 13- by 9-inch casserole dish or baking pan. Mix the breadcrumbs with the remaining cheese and sprinkle over the top of casserole.
Bake until bubbly and lightly browned, 20 minutes.