From Amy Green of Simply Sugar and Gluten-Free
Preheat the oven to 375 degrees.
Drizzle oil and add 1 T. salt to a large pot of boiling water. Add the macaroni and cook according to the directions on the package, 5-8 minutes. Drain well and rinse pasta with cold water.
Meanwhile, heat the milk in a small saucepan, but don’t boil it. Melt 3 T. of butter in a large (4-quart) pot and add the gluten-free flour. Cook over low heat for 2 minutes, stirring with a whisk. While whisking, add the hot milk and cook for a minute or two more, until thickened and smooth. Off the heat, add the Gruyere, Cheddar, 1 T. salt, pepper, and nutmeg. Add the cooked macaroni and stir well. Pour into a 1 1/2-quart baking dish.
Slice the tomatoes and arrange on top. Melt the remaining 1 T. of butter, combine with the bread crumbs, and sprinkle on the top. Bake for 30-35 minutes, or until the sauce is bubbly and the macaroni is browned on the top.
Note: You can make this dish ahead of time – just add the tomatoes and bread crumbs right before putting it into the oven.