Green Bean Casserole with Caramelized Onions

To make onion topping: Heat oil in a large nonstick skillet over low heat. Add sliced onion and cook, stirring occasionally, until very tender and golden, about 30 minutes. Set aside.

Meanwhile, preheat oven to 350 degrees F. Spread breadcrumbs on a baking sheet and toast, stirring once, until lightly browned, 5 to 10 minutes. Set aside.

To make sauce: Combine milk, peppercorns, bay leaf and nutmeg in a medium saucepan and heat over low until steaming. Remove from heat, let stand for 5 minutes and strain into a measuring cup. (Discard peppercorns and bay leaf.)

Meanwhile, heat oil in a large saucepan over medium heat. Add chopped onion and cook, stirring often, until golden, 3 to 4 minutes. Add mushrooms and garlic and cook, stirring, until tender, 3 to 4 minutes. Sprinkle flour over the vegetables and cook, stirring, for 1 minute. Slowly pour in the milk, whisking constantly. Bring to a boil, stirring. Reduce heat to low and cook, stirring, until thickened, about 1 minute. Remove from heat. Whisk in sour cream, salt and pepper.

To assemble and bake casserole: Preheat oven to 425 degrees F. Spread green beans evenly over the bottom of a shallow 2-quart baking dish and pour the sauce over the top. Toss together the reserved onions and breadcrumbs in a small bowl and spread over the beans. Bake until bubbling, 15 to 25 minutes.