Preheat the oven to 400F. Line a sheet pan with parchment paper
Transfer the mushrooms, bell peppers, and onion to a sheet pan, then mix with olive oil and fajita seasoning (homemade or store-bought taco seasoning mix)
Spread vegetables evenly and bake for 15-20 minutes, or until they all turn a nice brown color
Squeeze fresh lime juice over the vegetables and sprinkle with fresh cilantro