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1
Put oven rack in lower third of oven and put a large shallow baking pan on rack, then preheat oven to 475u00b0F.
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2
Toss one third of squash with 1 tablespoon oil, 1/2 teaspoon salt, and 1/2 teaspoon pepper in a bowl, then spread in preheated baking pan in 1 layer and roast in oven, stirring once, until tender, 12 to 15 minutes. Transfer squash to a large bowl. Roast remaining squash in 2 batches in same manner, tossing with 1 tablespoon oil, 1/2 teaspoon salt, and 1/2 teaspoon pepper (per batch) just before roasting and adding to bowl when done.
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3
Toss onions and bell peppers with remaining tablespoon oil, remaining 1/2 teaspoon salt, and remaining 1/2 teaspoon pepper in another large bowl, then spread in baking pan and roast, stirring once, until onions are golden, 8 to 10 minutes. Transfer to bowl with squash.
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4
Move oven rack to middle position and reduce oven temperature to 400u00b0F.
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5
Melt 3 tablespoons butter in a saucepan and remove from heat, then add bread crumbs and a pinch of salt, tossing to coat crumbs. Spread evenly in cleaned baking pan and bake, without stirring, until pale golden, about 5 minutes.
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6
Melt remaining 4 tablespoons butter in a 3-quart heavy saucepan over moderately low heat, then whisk in flour and cook roux, whisking constantly, 3 minutes. Add broth, whisking, and bring to a boil, whisking. Reduce heat and simmer, whisking occasionally, 3 minutes. Remove from heat and cool 5 minutes, whisking occasionally, then whisk in sour cream and salt and pepper to taste. Pour sauce over squash mixture and stir gently until combined well.
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7
Butter a 13- by 9- by 2-inch glass or ceramic baking dish (3-quart capacity), then spread squash mixture evenly into it and sprinkle with bread crumbs. Bake casserole until golden and bubbling, 15 to 20 minutes. Serve immediately.
