Preheat oven to 375 degrees.
Bring a large pot of water to a rolling boil. Cook pasta for one minute less than package directions state. Drain.
Heat the milk and cream in a small saucepan, but don't let it boil.
Melt 3 tablespoons of butter in a medium sized pot and add the flour. Cook, whisking constantly, over low heat for 2 minutes.
Continue whisking and slowly pour in the hot milk and cream. Cook for another 2 minutes until thickened and smooth.
Remove from the heat and stir in the Gruyere, Cheddar, salt, pepper, and nutmeg.
Add the cooked macaroni and stir to coat.
Pour into a pie plate or other dish of similar size.
Slice the tomatoes and arrange on top.
Melt the remaining 1 tablespoon of butter and mix it with the bread crumbs, Parmesan cheese, and garlic powder. Sprinkle on top of the tomatoes.
Bake for 25 minutes or until the sauce is bubbly and the bread crumbs are brown and crispy.