Place sausage in an oven-safe pot or Dutch oven over medium heat; cook and stir until crumbled and browned, about 5 minutes. Transfer sausage to serving plate, reserving drippings in the pot. Add onion; cook and stir until softened, about 5 minutes. Add garlic; cook and stir until fragrant, about 3 minutes.
Stir sausage, cannellini beans, mushrooms, and chicken broth into the pot. Increase heat and bring to a boil. Remove from heat. Place all broccoli rabe on top.
Bake in the preheated oven, covered, until broccoli rabe wilts, about 15 minutes. Remove from oven; stir well and season with salt and pepper. Continue baking until flavors combine, about 15 minutes more.
