Sauté Vegetables. Preheat the oven to 375°F (190°C). Heat olive oil in a cast-iron pan or large skillet over medium-high heat. Add the chopped onion and green pepper and cook, stirring occasionally, for 6 to 8 minutes until tender.
Brown Meat. Add the ground beef and mushrooms to the pan. Cook, breaking up the beef with a spoon, until fully browned. Carefully drain excess fat if necessary.
Combine Base. Stir in the cooked rice, tomatoes, and spices until evenly mixed, then turn off the heat. Sprinkle ½ cup of cheese over the mixture and stir until fully melted and incorporated. Remove the skillet from heat.
Prep and Carve Peppers. Slice the tops off the peppers using a paring knife and set them aside as lids. Core and seed the peppers, then use the paring knife to carve a jack-o'-lantern face into one side of each pepper.
Fill and Bake. Stand the peppers upright in a 9x13-inch baking dish, spoon the beef mixture inside to the top, and replace the pepper lids. Pour ¼ cup of warm water into the bottom of the dish, cover tightly with aluminum foil, and bake at 375°F (190°C) for 35 minutes until the peppers are nearly fork-tender.
Add Remaining Cheese. Remove the baking dish from the oven. Uncover the dish, carefully remove the aluminum foil, and discard it. Remove the pepper lids and set them aside. Sprinkle the remaining cheese evenly over the top of each pepper. Return uncovered to the oven and bake for 5 minutes, until the cheese is melted and bubbly.
Garnish and Serve. Sprinkle with freshly chopped cilantro, if desired. Use tongs to carefully transfer the warm peppers from the pan to serving plates and enjoy!