Use a 6-quart slow cooker. Brown the meat on the stovetop in a large skillet along with the red onion. Drain any accumulated fat and pour pan contents into the slow cooker insert. Add pasta sauce, mushrooms, bell pepper, Italian seasoning, and beef broth. Stir in the broken lasagna noodles.
Cover and cook on low for 7-8 hours, or until fully hot and pasta has reached desired consistency. Serve in large bowls with a small bowl of shredded cheese per person to stir in at the table.