Combine the soy sauce, balsamic vinegar, garlic powder and oregano in a small bowl. Season with pepper. Pour the mixture over the prepared mushrooms and toss to combine.
Heat the olive oil in a large nonstick sauté pan over medium-high heat. When the oil is hot, add the mushrooms (with any liquid), peppers and onions. Cook until the mushrooms have released their liquid and the mushrooms and onions begin to brown, about 10 minutes.
Lay the cheese over the mushroom mixture, cover and cook just until the cheese melts, 1 to 2 minutes. Then give everything a mix. Remove from the heat, spoon into the butter lettuce leaves and serve.