In a 3 to 4-quart heavy saucepan cook yellow peppers, onion, and garlic in 1 tablespoon oil, covered, over moderately low heat, stirring occasionally, until peppers are soft, about 10 minutes. Stir in cream. In a blender puree mixture in batches (use caution when blending hot liquids), transferring to a large bowl, and season with salt and pepper. (Sauce may be made 2 days ahead and chilled, covered. Bring sauce to room temperature before proceeding.) In a deep large heavy skillet cook mushrooms and peppers in remaining 2 tablespoons oil over moderately high heat, stirring occasionally, until peppers are softened and most of liquid is evaporated, about 5 minutes, and season with salt and pepper.
Preheat oven to 375 degrees.
Fill a 6 to 7-quart kettle 3/4 full with salted water and bring to a boil. Stir in ziti and cook until just tender, about 10 minutes. Reserve 1 cup pasta cooking water and drain ziti well in a colander. Rinse ziti and drain well. Into sauce stir ziti, mushroom mixture, scallion greens, 1 1/2 cups Parmesan, reserved cooking water, and salt and pepper to taste. Spoon mixture into a 3-quart shallow baking dish and sprinkle with remaining 1/2 cup Parmesan. Bake ziti in middle of oven until hot and pasta begins to brown, about 15 minutes.
Recipe courtesy of Gourmet Magazine