Sauté the vegetables. In a cast-iron pan or large skillet, heat olive oil over medium-high heat. Add the diced onion and green pepper. Cook, stirring occasionally, for 5 to 8 minutes, or until the vegetables become tender.
Brown the meat. Add the ground beef and diced mushrooms to the pan. Cook, breaking up the beef with a wooden spoon, until the beef is fully browned. If there's a lot of grease in the pan, carefully drain any excess fat from the meat mixture before moving to the next step.
Add the rice, tomatoes, and spices. Stir in the cooked rice, drained fire-roasted tomatoes, and all the spices into the meat and vegetable mixture. Continue stirring until everything is evenly combined. Once mixed, turn off the burner.
Add the cheese. Sprinkle ¼ cup of cheese over the beef mixture in the skillet. Stir until the cheese is melted and evenly incorporated. Once combined, set the skillet aside.
Prep and carve bell peppers. With a sharp paring knife, carefully cut off the tops of each bell pepper and set them aside. You'll use these as lids later. Then, reach inside each pepper and remove and discard the core and all the seeds. Finally, carefully carve a jack-o'-lantern face into one flat side of each pepper half. Get creative with your spooky or silly designs!
Fill the peppers and bake. Arrange the carved bell peppers upright in a 9x13-inch baking dish. Spoon the beef and rice mixture into each pepper, filling them about three-quarters full. This leaves some space at the top for the cheese you'll add later. Place the reserved pepper tops (stems) back onto the filled peppers as lids. Bake the peppers uncovered in the oven for 30 minutes.
Top with cheese. After baking for 30 minutes, carefully take the baking dish out of the oven. Set the pepper tops (stems) aside. Sprinkle the remaining cheese evenly over the top of the filling in each pepper. Place the baking dish back in the oven for another 5 minutes, or until the cheese is melted, bubbly, and slightly golden.
Garnish. Serve with freshly chopped cilantro.