In a large pan, heat up one tablespoon olive oil. Sautee the onion and garlic for a few minutes. Add the peppers and the mushrooms. Cook for about 10 minutes and add the spices. When the vegetables are ready, combine with the cooked rice and adjust seasoning if needed.
Preheat the oven to 400°F.
Pour the tomato sauce into a large baking dish. Stuff the peppers with the rice mixture and bake for about 30 to 35 minutes.