1. Cut a thin slice from top of each pepper; scoop out seeds and membrane. Parboil peppers in a small amount of boiling salted water for 3 minutes; drain well. Stand peppers in slow cooker.
2. Shape ground beef into a large patty in a skillet; brown 5 minutes on each side; then break up into small chunks.
3. Stir in pork and beans, half of the onion rings and mustard. Spoon into pepper cups, divinding evenly; pour tomato sauce over ; cover.
4. Cook on low for 8 hours or on high for 4 hours, or until peppers are tender. Top with remaining French fried onion rings.