Cook ziti according to pasta directions; drain and set aside.
Heat the oven to 350 F.
Grease a 2 1/2-quart casserole.
In a skillet over medium heat, brown the sausage. Remove to paper towels to drain. Wipe the pan out with paper towels.
Place the skillet over medium-low heat and add the butter. When the butter has melted and stopped foaming, add the mushrooms, green bell pepper, and onion; cook the vegetables until just tender.
Blend the flour into the vegetable mixture, stirring until well blended and bubbly. Continue cooking for 1 to 2 minutes, stirring constantly.
Gradually add the milk and cook until the mixture begins to boil, stirring constantly.
Continue to cook for 1 minute, stirring frequently. Add 1 1/2 cups of cheddar cheese, the Parmesan cheese, and the pepper. Stir until the cheese has melted and the mixture is smooth.
Stir the hot cooked ziti and sausage into the sauce.
Pour the pasta mixture into the prepared casserole.
Top the pasta with the remaining 1/2 cup cheddar cheese.
Bake in the preheated oven for 30 minutes or until hot and bubbly.