Preheat oven to 375 degrees F.
Remove the stems from the mushrooms (you should be able to pop them out with your fingers). Reserve 6 of the stems. Discard the rest.
Coat a baking sheet with cooking spray or oil. Lay the mushrooms, cavity side down, on the baking sheet and bake until the mushrooms are just tender, about 10 minutes. Turn over the mushrooms so that the cavity is facing up.
Trim, then dice the reserved mushroom stems.
Heat olive oil in a medium skillet set over medium heat. Add chopped mushroom stems, garlic and red bell pepper. Cook, stirring frequently, until mushrooms are soft, 4 to 5 minutes.
Remove the skillet from the heat and stir in the black beans, brown rice, cilantro, salsa and 1 tablespoon of the queso fresco cheese.
Divide the mixture between the mushrooms, gently stuffing it into the open cavities.
Sprinkle the remaining queso fresco cheese over the stuffed mushrooms.
Bake the mushrooms until the cheese is starting to melt and turn brown, 8 to 10 minutes.