Serve 6 to 8
4 tablespoons butter, divided
3 large leeks, dark green leaves trimmed, light green and white part quartered lengthwise, sliced thin, and thoroughly rinsed and drained
Salt and ground black pepper
1 tablespoon flour
1/2 cup chicken or vegetable broth
1/2 cup milk
2 cups grated extra-sharp grated cheese
1/2 cup dry bread crumbs
1/4 cup grated Parmesan cheese
1/2 teaspoon dried thyme leaves
Adjust oven rack to lower-middle position and heat oven to 400 degrees. Melt 2 tablespoon of the butter in a microwave-safe 2-quart casserole (such as a 9-by 9-inch pan) Add leeks, along with a generous sprinkling of salt and pepper; toss to coat. Bake, stirring at 15 minutes, until tender and most of the liquid has evaporated, about 20 minutes. Leave oven at 400 degrees.
Meanwhile, heat another tablespoon of the butter in a large saucepan over medium-high heat. Whisk in flour and then broth and milk; continue to whisk until sauce thickens, a couple of minutes. Stir in cheddar, and then stir sauce into leeks.
Mix breadcrumbs, Parmesan, thyme, and remaining 1 tablespoon of butter (melted); sprinkle evenly over leeks. Bake until leeks are bubbly and breadcrumbs are golden brown, about 15 minutes. Serve.