Three Cheese Baked Ziti

1 package (16 oz.) ziti pasta, cooked
1 medium onion, finely chopped
1/2 cup butter
2 tablespoons instant chicken boullion
3 tablespoons flour
6 cups milk
8 oz. Cheddar cheese
8 oz. American cheese
1/2 cup fresh Parmesan cheese
1/2 cup breadcrumbs, optional

Preheat oven to 350 degrees F. In a large saucepan, melt butter, add onion, saute until tender. Add instant boullion, stir until dissolved. Stir in flour to make a roux. Cook for 1 to 2 minutes until browned. Slowly whisk in milk, until no lumps are apparent. Add all 3 cheeses (grated), stir over low heat until completely melted. Put cooked pasta into a 2 quart casserole dish and pour sauce over. Sprinkle with bread crumbs (and additional Parmesan cheese if desired). Bake 30 to 40 minutes, or until brown and bubbly.