1 head cauliflower or romanesco4 tablespoons olive oil Kosher salt and pepper to taste 3 cloves garlic, minced 2 leeks 1 1/2 tablespoons butter 1 1/2 tablespoons flour 2 cups milk 1/2 cup (generous) grated Parmesan, Gruyere or Manchego cheese 1 teaspoon whole-grain Dijon mustard 1/4 cup breadcrumbs
Preheat the oven to 400°F. Cut the cauliflower or romanesco into florets that are 1 1/2 inches wide. In a large bowl, toss the florets with 3 tablespoons olive oil. Season with salt and pepper, and spread out on a large rimmed baking sheet. Roast until tender with just a bit of bite and browned, 35 to 40 minutes, stirring occasionally. Stir in the garlic after 20 minutes.
Meanwhile, remove the coarse dark tops of the leeks and cut lengthwise. Wash thoroughly, then cut each half into 2-inch lengths, discarding any more dark green ends and checking for hidden dirt. Separate the layers 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.)
In a medium saucepan, melt the butter over medium-low heat. Whisk in the flour and cook, stirring constantly, for 3-5 minutes, taking care that it doesn't brown. Whisk in the milk until smooth, then bring to a simmer. Maintain the heat so it's bubbling gently; stir often until the sauce thickens and the flour cooks, about 15 minutes. Add 1/3 cup of the Parmesan 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°. Toss the vegetables in a 13- by 9-inch baking pan, such as a Pyrex. Pour the sauce evenly over the vegetables. Combine the breadcrumbs with the remaining cheese and sprinkle over the top.
Bake until bubbly and lightly browned, 20 minutes. If you like, run the gratin briefly under the broiler to brown well. Serve warm.