Preheat oven to 350℉.
Cut top of peppers (I chop and keep this part of the peppers for a later recipe to avoid waste). Remove and discard seeds and membranes.
Bring a large pot of water to boil, and cook the peppers for 4 minutes. Remove peppers and drain well.
Arrange peppers in a 9x13 baking dish.
Heat olive oil in a skillet, cook the onions, garlic and mushrooms over medium-heat until softened.
Transfer skillet mixture to a large bowl, add cooked rice, black beans, diced tomatoes, chili powder, cumin, salt and pepper. Mix well. Gently mix in 1½ cup of cheese.
Spoon rice mixture into the peppers, sprinkle remaining cheese on top.
Bake about 30 minutes, until cheese is bubbling.