While pasta is cooking, saute onion and sausage until onion is soft. Add frozen veggies and cook about five minutes more.
After pasta is cooked combine with sausage mixture.
Make the cheese sauce by melting the butter. Once melted add the flour and cook slightly to make a roux. Slowly add cream and stir until thickened. Add two cups cheese and stir until melted.
Mix the cheese sauce in with the pasta and sausage, and top with bread crumbs and 1/4 cup cheese, and bake at 350 for 30 minutes.