GREEN-BEAN CASSEROLE WITH CHESTNUTS AND BUTTERED BREADCRUMBS

Steps:

• Preheat oven to 350°F. Generously brush a 3-quart baking dish with butter. Prepare a bowl of salted ice water. Cook green beans in a large pot of generously salted boiling water until crisp-tender and dark green, 3 to 4 minutes. Drain; immediately transfer to ice water until thoroughly chilled. Drain and pat dry. Halve beans on the bias and transfer to prepared dish.
• In a bowl, toss breadcrumbs with chestnuts to combine. Melt 6 tablespoons butter; drizzle evenly over breadcrumb mixture, tossing until evenly absorbed. Toss in parsley; season with salt and pepper.
• In a small saucepan, heat milk and cream over medium-low until scalding and bubbles emerge along edges of pan. Remove from heat; cover to keep warm.
• Sprinkle garlic with a pinch of salt; mash into a paste with the side of a knife. In a large straight-sided skillet, melt remaining 6 tablespoons butter over medium heat. Add mixed mushrooms; season with salt. Cook, stirring occasionally, until tender and beginning to brown, 15 to 18 minutes. Add onion, season with salt, and cook, stirring often, until onion is translucent but not browning, about 5 minutes.
• Stir in garlic; cook 1 minute more. Sprinkle flour evenly over mixture and cook, stirring, 2 minutes. Slowly add milk mixture, stirring, and bring to a simmer. Cook until thickened slightly, about 3 minutes. Stir in nutmeg; season with salt and pepper. Pour mixture evenly over green beans in dish. Sprinkle breadcrumb mixture evenly over top.
• Bake until bubbly along edges and crisp golden brown on top, about 30 minutes. Meanwhile, toss sliced trumpet mushrooms with enough oil to lightly coat, season with salt and pepper, and arrange in a single layer on a rimmed baking sheet. Roast until tender and golden brown along edges, about 30 minutes. Top casserole with roasted mushrooms and serve warm.