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1
Chop yellow bell peppers and onion and mince garlic.
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2
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 30 minutes.
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3
Stir in cream.
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4
In a blender puree mixture in batches (use caution when blending hot liquids), transferring to a large bowl, and season with salt and pepper.
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5
(Sauce may be made 2 days ahead and chilled, covered.
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6
Bring sauce to room temperature before proceeding.)
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7
Thinly slice mushrooms lengthwise and cut red and orange bell peppers into 1/4-inch-wide strips.
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8
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.
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9
Preheat oven to 375F.
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10
Fill a 6- to 7-quart kettle three fourths full with salted water and bring to a boil.
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11
Stir in ziti and cook until just tender, about 10 minutes.
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12
Reserve 1 cup pasta cooking water and drain ziti well in a colander.
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13
Rinse ziti and drain well.
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14
Thinly slice scallion greens.
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15
Into sauce stir ziti, mushroom mixture, scallion greens, 1 1/2 cups Parmesan, reserved cooking water, and salt and pepper to taste.
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16
Spoon mixture into a 3-quart shallow baking dish and sprinkle with remaining 1/2 cup Parmesan.
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17
Bake ziti in middle of oven until hot and pasta begins to brown, about 20 minutes.
