Mac & Cheese Gratin

8 Tablespoons (1 stick) unsalted butter, plus more for dish
6 slices good white bread, crusts removed, torn into 1/2 inch pieces
5 1/2 cups milk
1/2 cup all-purpose flour
2 teaspoons kosher salt
1/4 teaspoon grated nutmeg
1/4 teaspoon freshly ground black pepper
1/4 teaspoon cayenne pepper, or to taste
4 1/2 cups grated sharp cheddar cheese (note: Tillamook or Cabot is best)
1 1/4 cups grated Pecorino Romano cheese

1 pound large elbow macaroni
3 hothouse tomatoes, sliced

Heat oven to 375 degrees. Butter a 3-quart casserole dish; set aside. Place bread in a medium bowl. In a small saucepan over medium heat, melt 2 Tablespoons butter. Pour butter into bowl with bread, and toss. Set breadcrumbs aside.
In a medium saucepan set over medium heat, heat milk (note: this step is important). Melt remaining 6 Tablespoons butter in a high-sided skillet over medium heat. When butter bubbles, add flour. Cook, whisking, 1 minute.
While whisking, slowly pour in hot milk. Continue cooking, whisking constantly, until the mixture bubbles and becomes thick (note: cook over medium-low heat. I think the sauce will be more smooth if you don't rush the thickening process.)
Remove pan from heat. Stir in salt, nutmeg, black pepper, cayenne pepper, 3 cups cheddar cheese, and 1 cup Pecorino Romano; set cheese sauce aside.
Fill a large saucepan with water; bring to a boil. Add macaroni; cook 2 to 3 minutes less than manufacturer's directions, until the outside of pasta is cooked and the inside is underdone. (Different brands of macaroni cook at different rates; be sure to read the instructions.) Transfer macaroni to a colander, rinse under cold running water, and drain well. Stir macaroni into the reserved cheese sauce.
Pour mixture into prepared dish. Sprinkle remaining 1 1/2 cups cheddar cheese, 1/4 cup Pecorino Romano, and breadcrumbs over top.
Lay sliced tomatoes on a cookie sheet. Drizzle with olive oil, coarse salt, freshly ground pepper and thyme leaves. Broil for about 10 minutes until softened. Let cool for a few minutes and then arrange on top of the gratin (over the breadcrumbs).
Bake until browned on top, about 30 minutes. Transfer dish to a wire rack to cool 5 minutes; serve hot.