Ingredients

• 4 lb yellow squash (10 medium), trimmed, halved lengthwise, and sliced crosswise 1/8 inch thick
• 1/4 cup vegetable oil
• 2 teaspoons salt
• 2 teaspoons black pepper
• 2 medium onions, chopped (2 cups)
• 1 green bell pepper, chopped
• 1 red bell pepper, chopped
• 7 tablespoons unsalted butter
• 4 slices firm white sandwich bread with crust, coarsely ground in a food processor (about 2 1/4 cups)
• 1/4 cup all-purpose flour
• 1 3/4 cups reduced-sodium chicken broth (14 fl oz)
• 1 cup sour cream

Instructions

1. Put oven rack in lower third of oven and put a large shallow baking pan on rack, then preheat oven to 475°F.
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.
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.
4. Move oven rack to middle position and reduce oven temperature to 400°F.
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.
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.
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.