Preheat oven to 375°F.
Prepare pasta according to package directions; drain. While pasta cooks, prepare the sauce. (Note: reserve 1/2 to 1 cup pasta water for the sauce.)
Heat oil over medium heat in a large saucepan (at least 3-quart). Add onion and cook about 3 to 5 minutes until softened. Add 2-1/2 cups milk and squash. Cover and simmer 15 minutes or until squash is tender and cooked through. Remove from burner and puree using an immersion blender OR potato masher. Return to medium heat. Whisk remaining 1/2 cup milk and corn starch in a small bowl until thoroughly combined; add to pan slowly. Bring mixture to a boil; boil for 1 minute.
Reduce heat to low. Add garlic powder, ground mustard, salt, pepper, cayenne pepper and cheeses. Stir until cheese melts; add cooked pasta. Stir in reserved pasta water if sauce is too thick.
Place in a 3-quart casserole OR 13 x 9-inch baking dish OR 8 individual casseroles that have been sprayed with cooking spray. Top with crumbs.
Bake for 20 to 25 minutes, until edges are bubbly.
Recipe Tips: Using a thawed, frozen 12 ounce box of cooked squash can be a timesaver. Stir squash into hot milk mixture, thicken and then add cheeses. This can easily be a gluten free recipe by using gluten free pasta and crushed gluten free rice square cereal.