In a bowl, mash beans until fairly smooth, but still chunky.
Mix in mushroom pieces, onion, egg, chilli powder, cumin, lime juice, salt and pepper.
Place the peppers, cut side up on a baking sheet lined with parchment.
Fill each pepper with bean mixture, mounding at the top.
Top each with cheese.
Bake in center of the oven for 15 - 20 minutes or until peppers have softened and cheese is melted. Remove from oven and garnish with cilantro. Serve immediately.