01 - Cook the Italian sausage or ground beef in a skillet over medium heat until browned. Drain excess fat and set aside.
02 - Add diced onion, bell pepper, and garlic to the skillet. Sauté for 3-4 minutes until softened and fragrant.
03 - Lightly grease the interior of your slow cooker with cooking spray or a small amount of oil.
04 - Transfer the cooked meat and vegetables to the slow cooker. Add uncooked pasta, marinara sauce, diced tomatoes with juices, Italian seasoning, red pepper flakes if using, salt, and pepper. Mix thoroughly to combine.
05 - Sprinkle half the shredded mozzarella over the pasta mixture. Arrange pepperoni slices evenly across the cheese layer.
06 - Cover and cook on LOW setting for 3.5 to 4 hours, or until pasta is tender. Check at the 3-hour mark to prevent overcooking.
07 - During the final 15 minutes of cooking, sprinkle the remaining mozzarella and Parmesan cheese over the top. Re-cover and allow cheese to melt completely.
08 - Serve hot while cheese is melted and bubbly. Garnish with additional Parmesan or fresh basil leaves if desired.