3. Gradually whisk in the milk and stir with a flat-edged wooden spoon or spatula, to make sure the flour doesn't stick to the bottom of the pan, until the sauce comes up to a simmer and thickens, a few minutes more.
5. Add the onion to the sauce and stir until heated through, 1 to 2 minutes. Transfer the sauce to a blender and process until pureed, or puree in the pan with a hand blender.
6. Keep the sauce warm over low heat. You should have about 2 cups soubise.
3 tablespoons butter, melted and divided
3/4 pound macaroni, penne or other short tube pasta
1 pound Comté, Gruyère, Emmentaler, cheddar or a combination of these cheeses, grated and divided
1/4 cup grated Parmigiano-Reggiano (about 1 ounce)
1/2 cup panko bread crumbs
1. Brush the inside of a 9-inch by 13-inch baking dish with 1 tablespoon of the butter. Cook the pasta just until al dente, drain, then place in a large bowl.
2. Sprinkle half of the Comté cheese into the soubise and stir until melted. Remove from the heat and pour over the pasta. Toss the pasta to mix well and pour it into the prepared baking dish. Top with the remaining Comté. (The dish can be prepared to this point up to 3 days before serving and refrigerated tightly covered.)
3. Heat the oven to 425 degrees. Sprinkle Parmigiano-Reggiano over the pasta. Stir the bread crumbs into the remaining melted butter and spread over top of the casserole. Cover with aluminum foil and bake until heated through, about 30 minutes (longer if it has chilled in the refrigerator).
4. Remove the foil and bake until the cheese is nicely browned, about 15 minutes. Serve immediately.
Each serving: 758 calories; 34 grams protein; 56 grams carbohydrates; 3 grams fiber; 43 grams fat; 25 grams saturated fat; 129 mg cholesterol; 5 grams sugar; 697 mg sodium.