Scalloped Squash, Red Pepper, and Corn Gratin with Cheddar Crust

PreparationFor sauce: Melt butter in heavy medium skillet over medium heat. Add flour and whisk 1 minute. Whisk in hot milk; whisk until mixture boils, thickens, and is smooth, about 5 minutes. Remove from heat. Add cheese and hot pepper sauce and whisk until cheese melts. Season sauce to taste with salt and pepper. For vegetables: Butter 11x7x2-inch glass baking dish. Heat 1/2 tablespoon oil in large nonstick skillet over medium-high heat. Add half of yellow squash and half of zucchini; sauté until crisp-tender, about 4 minutes. Transfer squash-zucchini mixture to large bowl. Repeat with 1/2 tablespoon oil and remaining yellow squash and zucchini. Heat remaining 1 tablespoon oil in same skillet over medium-high heat. Add onion, bell pepper, and garlic; sauté until onion is golden and pepper is tender, about 7 minutes. Stir in corn and rosemary. Transfer to bowl with squash-zucchini mixture. Mix in cheese sauce. Season to taste with salt and pepper. Transfer mixture to prepared dish. Dot with butter. (Can be prepared 1 day ahead. Cool slightly. Cover with foil and refrigerate. Before continuing, heat covered gratin in 400°F oven until warm but not hot, about 20 minutes.) For topping: Preheat oven to 400°F. Mix breadcrumbs, cheese, and melted butter in bowl until crumbs are coated with butter. Sprinkle crumbs evenly over gratin. Bake until vegetables are heated through and crumbs are crisp and golden, about 35 minutes. Test-kitchen tip:To make fresh breadcrumbs, cut bread into two-inch pieces and place in processor. Blend until bread is ground but some coarse crumbs remain.

PreparationFor sauce: Melt butter in heavy medium skillet over medium heat. Add flour and whisk 1 minute. Whisk in hot milk; whisk until mixture boils, thickens, and is smooth, about 5 minutes. Remove from heat. Add cheese and hot pepper sauce and whisk until cheese melts. Season sauce to taste with salt and pepper. For vegetables: Butter 11x7x2-inch glass baking dish. Heat 1/2 tablespoon oil in large nonstick skillet over medium-high heat. Add half of yellow squash and half of zucchini; sauté until crisp-tender, about 4 minutes. Transfer squash-zucchini mixture to large bowl. Repeat with 1/2 tablespoon oil and remaining yellow squash and zucchini. Heat remaining 1 tablespoon oil in same skillet over medium-high heat. Add onion, bell pepper, and garlic; sauté until onion is golden and pepper is tender, about 7 minutes. Stir in corn and rosemary. Transfer to bowl with squash-zucchini mixture. Mix in cheese sauce. Season to taste with salt and pepper. Transfer mixture to prepared dish. Dot with butter. (Can be prepared 1 day ahead. Cool slightly. Cover with foil and refrigerate. Before continuing, heat covered gratin in 400°F oven until warm but not hot, about 20 minutes.) For topping: Preheat oven to 400°F. Mix breadcrumbs, cheese, and melted butter in bowl until crumbs are coated with butter. Sprinkle crumbs evenly over gratin. Bake until vegetables are heated through and crumbs are crisp and golden, about 35 minutes. Test-kitchen tip:To make fresh breadcrumbs, cut bread into two-inch pieces and place in processor. Blend until bread is ground but some coarse crumbs remain.