01 - In a skillet over medium heat, cook the Italian sausage or ground beef until browned. Drain excess fat.
02 - Add the diced onion, bell pepper, and garlic to the skillet. Sauté 3-4 minutes until softened.
03 - Lightly grease the inside of your slow cooker.
04 - Add the cooked meat and vegetables to the crockpot. Stir in uncooked pasta, marinara sauce, diced tomatoes with juices, Italian seasoning, red pepper flakes if using, salt, and pepper. Mix well.
05 - Top with half the shredded mozzarella, then arrange pepperoni slices over the cheese.
06 - Cover and cook on LOW for 3.5 to 4 hours, or until pasta is tender. Check at 3 hours to avoid overcooking.
07 - In the last 15 minutes, sprinkle with the remaining mozzarella and Parmesan cheese. Re-cover and let the cheese melt.
08 - Serve hot, garnished with extra Parmesan or fresh basil if desired.