Brussels Sprouts and Mushroom Casserole

Ingredients

• 6 tablespoons (3/4 stick) unsalted butter, divided
• 1 1/2 lbs Brussels sprouts, trimmed and halved
• 8 ounces button or cremini mushrooms, sliced
• 1 medium onion, chopped
• Salt and pepper
• 2 cloves garlic, minced
• 2 tablespoons all-purpose flour
• 1 cup chicken or vegetable stock
• 1 cup half and half
• 1 cup panko breadcrumbs
• 1 6-ounce can French fried onions