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