Preheat oven to 400ºF.

Slice bread in half length-wise; rub insides with garlic clove. Place bread, open-face, on one side of a baking sheet; spread onion on other side. Place broccoli on another baking sheet. Coat vegetables and bread with cooking spray. Bake until broccoli is tender, flipping bread over and stirring vegetables halfway through, about 15 to 20 minutes total. Remove pans from oven and cut bread into ½-inch cubes; reduce oven temperature to 350ºF.

Heat oil in a large nonstick skillet over medium-high heat. Remove sausage from casing; cook sausage, breaking it up with a wooden spoon as it cooks, about 7 to 10 minutes. Stir in broccoli and onion, bread cubes, broth, salt, pepper and red pepper flakes. (Make sure to scrape up any browned bits of food on sides of skillet and stir into mixture.)

Coat a 9-inch square baking dish with cooking spray. Spoon 3 cups broccoli mixture into prepared dish; sprinkle with ¼ cup cheese. Top with remaining broccoli mixture; sprinkle with remaining ½ cup cheese. Bake until cheese is melted and casserole is heated through, about 30 minutes. Slice into 8 pieces and serve.

Yields 1 piece per serving.