Bring a large pot of water to a boil over high heat. Meanwhile, cut top off peppers and remove seeds. Add several pinches of salt to boiling water, then add peppers and boil, using a spoon to keep them completely submerged, until brilliant green and their flesh slightly softened, about 3 minutes. Drain, set aside to cool.
Preheat oven to 350 degrees F. In a large skillet, cook the ground beef. When it is about 3/4 of the way through, add the chopped onion and let cook until it’s nearly translucent. Add the garlic before the onion if you want a more subtle flavor, or after if you want a stronger flavor. Next, add the cooked rice and stir. Open the cans of tomato sauce and diced tomatoes. Put half of each can into the skillet mixture and set the rest aside. Then add all of the dried seasonings and stir throughout. Let it simmer for one to two minutes, then add half of the cheese blend.
Once everything is combined well and the cheese is slightly melted, remove from heat. Place the bell peppers in a small dish with enough water in it to just cover the bottom. Spoon the skillet mixture evenly into each pepper, top with remaining cheese and place into the oven for 30 to 40 minutes.
In a small saucepan, add the remainder of the cans of tomato sauce and tomatoes. Warm over medium-low heat while the peppers cook. If desired, add some of the same spices used in the pepper filling. Once the peppers are out of the oven, spoon the sauce over each pepper and serve, with additional cheese if you’d like.