Preheat the oven to 350 degrees F (175 degrees C).
Bring a large pot of water and 1 tablespoon salt to a boil. Add green bell peppers and cook until slightly softened, 3 to 4 minutes. This will prevent the bell peppers from becoming crispy as they bake.
Drain bell peppers and arrange cut-side up in a 9x9-inch baking dish.
Heat olive oil in a skillet over medium heat. Add onion; cook and stir until softened, 5 to 10 minutes.
Transfer cooked onion into a large bowl; add rice, black beans, and tomatoes. Stir in chili powder, garlic salt, cumin, and 1/2 teaspoon salt until well combined. Fold in 1 1/2 cups Mexican cheese blend.
Spoon rice mixture evenly into bell peppers; sprinkle with remaining Mexican cheese blend.
Bake in the preheated oven until cheese is melted and bubbling, about 30 minutes.
