Preheat oven to 400F.
Heat the oil in a saute pan and saute the onions and garlic 3 minutes, until soft.
Add the cumin, mushrooms, green chiles, and salt. Cook an additional 3-4 minutes, until the mushrooms are soft.
Place the mushroom mixture in a large mixing bowl and add the beans, tomatoes, cilantro, brown rice, salsa, and cheese. Stir to combine.
Place 3/4 to 1 cup of the filling in the center of a tortilla, fold the sides in, then fold the bottom up to cover the filling, and fold the top down.
Place the chimichangas, folded side down, on a greased baking sheet, and brush or spray them lightly with oil.
Bake 15 minutes, until the tortillas are golden brown. Serve with sliced avocado, sour cream, extra salsa and cilantro.
Note: If the tortillas are too stiff to fold, place them in the microwave for 30 seconds between 2 damp paper towels to soften.