In a large deep skillet over medium high heat brown ground beef and onion. Drain off any excess liquid. Add bell pepper, basil, oregano, pepperoni, and pizza sauce. Reduce heat and simmer for 20 minutes.
Spoon the mixture onto the buns and sprinkle with the mozzarella cheese. If you want the cheese to melt better, you can microwave the sandwiches for a few seconds.
These can also be cooked in the slow cooker. Just follow step one by browning the ground beef in and onion in a pan. Then drain the mixture of any excess fat and then pour it into a lightly greased slow cooker. Add the rest of the ingredients (except for the buns), cover, and cook on low for 4-5 hours.