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 tablespoon oil, covered, over moderately low heat, stirring occasionally, until peppers are soft, about 15 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.)
Thinly slice mushrooms lengthwise and cut red and orange bell peppers into
1/4-inch wide strips. 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 8 minutes, and season with salt and pepper.
Preheat oven to 375 degrees F.
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. Thinly slice scallion greens into sauce and stir in 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 20 minutes.
* When blending hot liquids: Remove liquid from the heat and allow to cool for at least 5 minutes. Transfer liquid to a blender or food processor and fill it no more than halfway. If using a blender, release one corner of the lid. This prevents the vacuum effect that creates heat explosions. Place a towel over the top of the machine, pulse a few times then process on high speed until smooth.
Recipe courtesy of Gourmet Magazine