In a 4 quart saucepan, brown sausage, remove and drain.
Melt butter and cook mushrooms, peppers and onion until tender, but not brown. Blend flour into vegetable mix; gradually stir in mil. Cook stirring constantly over medium low heat until mixture boils. Boil and stir one minute.
Blend in 1 1/2 cups cheddar cheese, Parmesan cheese and pepper until cheeses are melted and mix is smooth.
Cook Ziti and drain.
Stir hot cooked ziti and sausage into vegetable cheese mixture. Pour into a butter 2 quart casserole. Sprinkle with remaining 1/2 cup cheddar cheese.