1 pound rigatoni pasta (or elbow macaroni)
1 tablespoon vegetable oil
1/2 cup chopped onion
2 tablespoon butter
1 1/2 cups skim milk
2 tablespoon all-purpose flour
1/4 cup shredded Gruyere cheese
1/4 cup shredded Pecorino Romano cheese
3/4 cup shredded light white cheddar cheese
1/4 teaspoon salt
1/2 cup Panko bread crumbs
Preheat oven to 375 degrees F. Bring a large pot of water to boil, then add pasta and cook for about 5 minutes (al dente). Strain and rinse with cool water, then set aside.
Heat the oil in a skillet over medium heat, then cook the onions for about 5 minutes.
In a separate saucepan, melt the butter and add the salt and flour. Stir well. Add the milk and heat until bubbles begin to form, stirring occasionally. Add the onion and cheeses, and continue to stir over low heat with a whisk until the sauce thickens.
Transfer pasta to a greased or non-stick 7.25 baking pan and pour sauce over. Top with Panko break crumbs and bake for 25 minutes.