In large skillet over medium heat, sauté bacon until crispy; remove to paper towels. Drain off all but 2 tablespoons drippings.
Add onion and garlic to skillet; salt and pepper to taste. Sauté until onions are softened. Whisk in flour; cook 1 minute. Whisk in milk and cayenne pepper; cook until sauce has thickened, whisking constantly (about 3-4 minutes). Remove from heat.
Stir in green beans, cheese and bacon; transfer to a greased 2-quart baking dish. Toss bread crumbs with melted butter; sprinkle over casserole.
Bake in a preheated 350°F oven 30 minutes or until bubbly and golden brown.