Preheat oven to 400 degrees. Gently take the stems out of the mushrooms and discard. Place the mushrooms gill side up on a baking sheet covered with foil and lightly sprayed with non-stick cooking spray. Lightly sprinkle salt & pepper over the mushrooms.
In a medium sized bowl, combine the ricotta & mozzarella cheeses. Add the Pancetta and diced red bell pepper. Mix well.
Using a teaspoon, fill up the mushrooms with the cheese mixture. Bake for approximately 20 minutes or until mushrooms appear to be tender. Remove from oven.
Place mushrooms on a serving dish and garnish with green onion.