Preheat your oven to 375℉. Spray a 9x13 baking dish with cooking spray.
Bring a large pot of water to a boil and cook the elbow pasta according to the package directions. Strain and set aside.
1 pound elbow pasta
While the pasta is cooking, add 6 tablespoons of butter and flour to a medium sized pot and heat over medium heat. Whisk the melted butter and flour together then whisk in the warm milk. Whisk over the heat for about 1-2 minutes so the mixture gets a little bit thick.
½ cup butter, ½ cup all purpose flour, 4 cups whole milk
Remove the pot from the stovetop and add both grated cheeses, the salt, pepper and nutmeg. Whisk together so the cheese melts into the hot milk.
4 cups grated Gruyere cheese, 2 cups grated sharp cheddar cheese, 1 teaspoon salt, ½ teaspoon ground black pepper, ½ teaspoon ground nutmeg
Add the cooked, drained pasta to the pot of cheese sauce and stir.
Pour the mac and cheese into a 9x13 baking dish and spread it across the pan so it is flat.
Slice the tomatoes and lay them on top of the mac and cheese, covering the top of the dish.
4 small tomatoes
In a small bowl, melt the remaining 2 tablespoons of butter and then stir in the breadcrumbs. Sprinkle the mix over the top of the mac and cheese.
1 ½ cups bread crumbs
Bake the mac and cheese for 30 minutes. The edges should be bubbling and the topping should be golden brown.
Scoop and serve while hot!