Preheat oven to 350 degrees F (175 degrees C). Grease a 2-quart casserole dish.
Put broccoli in a large microwave-safe bowl with a small amount of water in the bottom; cook in microwave until tender, about 5 minutes. Drain liquid from bowl. Stir mushroom soup, Cheddar cheese, and mayonnaise with the broccoli until evenly mixed; pour into prepared casserole dish.
Mix bread crumbs and melted butter together in a separate bowl; spread evenly over the broccoli mixture.
Bake in preheated oven until the surface is bubbling, 20 to 30 minutes.
