- 1 1/2 pounds green beans, trimmed
- 1 large sweet onion, thinly sliced
- 6 tablespoons butter, divided
- 2 (8-ounce) packages sliced cremini mushrooms
- 1 teaspoon salt, divided
- 1 clove garlic, minced
- 3 tablespoons all-purpose flour
- 2 cups milk, warmed
- 1 tablespoon chopped fresh thyme
- 1/2 teaspoon pepper
- 1/4 teaspoon nutmeg
- 1/2 cup panko-style breadcrumbs
Bring a large pot of water to a boil and blanch the green beans for 7 minutes. Shock the green beans in a bowl filled with ice water, then drain into a colander and set aside to dry.
Heat 2 tablespoons butter in a large nonstick skillet. Sauté onion 10-12 minutes until caramelized. Remove from skillet and set aside.
Place the same pan back on the cooktop; melt another 2 tablespoons butter. Add the sliced mushrooms and season with 1/2 teaspoon salt. Sauté until mushrooms are soft and lightly caramelized. Stir in minced garlic; cook 2-3 minutes longer. Sprinkle flour over cooked mushroom mixture, stirring until well combined. Gradually pour in warm milk while stirring. Heat to a low simmer and cook until sauce begins to thicken. Add about 1/4 cup of the caramelized onions to sauce and stir in remaining 1/2 teaspoon salt, pepper and nutmeg. Add green beans and stir to coat.
Melt remaining 2 tablespoons butter in microwave-safe bowl, gently stir in breadcrumbs until butter is absorbed; set aside.
Pour green bean mixture into a cast-iron skillet or baking dish; top with remaining caramelized onions and breadcrumb mixture. Place under broiler for 8-10 minutes or until top is lightly browned. Serve immediately.