Broccoli Casserole

Preheat oven to 350°F. Lightly grease a 2.5 quart (or larger) baking dish with nonstick cooking spray and set aside.
Prepare broccoli as per package directions. Drain well and set aside.
32 ounces frozen broccoli florets
In a medium skillet, melt butter over medium heat. Add onion and cook, stirring often, for 3–4 minutes until softened. Stir in garlic and cook for 1 more minute.
2 Tablespoons unsalted butter, ½ medium yellow onion, 1 clove garlic
Transfer the sautéed onion and garlic to a large mixing bowl. Add the soup, mayonnaise, egg, and black pepper. Stir until combined.
1 (10.5 ounce) can cream of mushroom soup, 1 cup mayonnaise, 1 large egg, ¼ teaspoon pepper
Fold in the shredded cheddar, mushrooms, and cooked broccoli.
12 ounces sharp cheddar cheese, 1 (4 ounce) can sliced mushrooms
Spread the mixture evenly into the prepared baking dish.
In a small microwave-safe bowl, melt remaining 2 Tbsp butter. Stir in the breadcrumbs until coated, then sprinkle evenly over the casserole.
2 Tablespoons unsalted butter, ½ cup panko bread crumbs
Bake uncovered for 35–45 minutes, or until the topping is golden and the edges are bubbly.
Let rest for 5 minutes before serving.