Preheat oven to 400°F Cover a baking tray or cookie sheet with aluminum foil.
Rinse bell peppers. Cut off the tops and remove the seeds and membranes to create a hollow inside. If necessary, trim off some of the bottom in order to get the hollowed bell peppers to sit upright on the cookie sheet.
Place the bell peppers upside down (bottom facing up and the open top facing downward) on the baking tray. Put the tray inside the oven for 30 minutes.
In the mean time, finely chop the garlic clove and medium onion. cook the rice with the liquid from the can of diced tomatoes, some water, the chopped garlic, and half the chopped onion. Remember that you won't need a lot of water because the liquid from the can of diced tomatoes will contribute to the cooking liquid. Remove the rice from heat after it is done cooking.
In another pan, cook the ground beef with the fajita seasoning (as directed on the fajita seasoning pkg). Before finishing the ground beef, add the remaining half of the chopped onion and can of drained diced tomatoes to the meat. Finish cooking the meat with the onion & tomato and then remove the meat from the heat.
Take the bell peppers out of the oven after 30 minutes and reduce the oven temperature to 350°F.
Carefully turn the bell peppers right side up so that the open tops are facing up.
Put about 3 tablespoons of the cooked rice mixture in the bottom of each bell pepper and smooth the rice layer down. Put another 2-3 tablespoons of the beef mixture in each of the peppers. sprinkle cheese on top of each. Repeat the layering process once more so that the peppers are stuffed and topped with cheese.
Place the baking tray with the bell peppers in the oven once more. Cook the peppers for another 15 minutes-- the cheese is bubbling and melted.
Remove the baking tray from the oven. Plate the stuffed bell peppers and enjoy!