In a medium skillet, cook ground beef and onion until beef is browned and onion is translucent. Add to a 4-6 quart slow cooker.
Add in broth, milk, pasta sauce, bell pepper, garlic, salt, Italian seasoning, and red pepper flakes and stir. Cover and cook on low for 6-8 hours or high for 3-4 hours. (*The cook time here is really flexible -- you want to cook the red pepper and get it hot enough to cook the pasta, so you can cook anywhere from 3 to 8 hours on low).
About 15 minutes before serving, turn the slow cooker to high and stir in macaroni. Check at 15 minutes, and add 5-10 extra minutes to the cook time as needed until the pasta is al dente. If the pasta is not cooked and most of the liquid is absorbed, add ½-1 cup additional broth or milk as needed.
Stir in cheese (covering for 2-3 minutes if necessary to melt the cheese), taste and serve.