Pre-heat oven to 400 degrees fahrenheit. Clean mushrooms, remove stems {save them for another recipe}, and place them in 1 or 2 large baking dishes.
Combine grated cheese, onions, bell peppers, olives, and tomato paste in a mixing bowl, and stir until well mixed.
Place a heaping teaspoon of cheese mixture in each mushroom, then bake in oven for ~20 minutes, or until cheese is melted and bubbly, and mushrooms are cooked. Allow to cool for 5-10 minutes, then transfer to a serving platter, and enjoy while still warm.