Boil sausage links in a saucepan with enough water to cover (boil for about 40 minutes). This removes much of the grease. Cool sausage and chop into 1/2 inch pieces. No need to remove casing.
Chop yellow bell peppers and onion and mince garlic. In a 3- to 4- quart heavy saucepan cook yellow peppers, onion and garlic in 1 tbl. oil, covered, over moderately low heat. Stir occasionally and cook until peppers are soft, about 30 minutes. Stir in the cream. In a blender, puree mixture in batches (use caution when blending hot liquids). Transfer blended sauce into a large bowl and season with salt and pepper.
Thinly slice mushrooms lengthwise and cut red and orange peppers into strips. In a deep large heavy skillet cook mushrooms and peppers in remaining 2 tbl of oil over moderately high heat, stirring occasionally, until peppers are softened and most of the liquid is evaporated, about 8 minutes. Season with salt and pepper and stir in sausage pieces.
Preheat oven to 375 degrees
Fill a 6- to 7- quart kettle with salted water and bring to a boil. Stir in ziti and cook until just tender, about 10 minutes. Reserve 1 cup of pasta cooking water and drain ziti well in colander. Rinse ziti and drain well. Thinly slice scallion greens. Into sauce stir ziti, mushroom mixture, scallions, 1 and 1/2 cups Parmesan, the reserved cooking water and salt and pepper to taste. Spoon mixture into a 3-quart shallow baking pan dish and sprinkle with remaining Parmesan cheese.
Bake until hot and pasta begins to brown, about 30 minutes.
Serve with garlic or focaccia bread