Preheat oven to 400°F. Line a baking sheet with parchment paper or a silicone baking mat.

Remove the stems from mushroom caps and slice a thin layer off the top of the mushroom cap to allow it to sit flat. (Keep these pieces)

Remove the gills and brush the mushrooms with olive oil and sprinkle with a little salt and pepper. Bake for 12 minutes.

Chop the mushroom stem and cap pieces that were trimmed in the first step.

In a large mixing bowl, combine the chopped mushroom pieces, chicken, pesto, roasted red pepper, onion, garlic, and ½ cup of the mozzarella cheese.

Divide the mixture evenly between the mushroom caps. Top with remaining mozzarella cheese.

Bake for 7 minutes more at 400°F. Then broil on high for 2 to 3 minutes or until the cheese is golden brown and bubbling.

Drizzle a little pesto over top of each mushroom. Garnish with Parmesan cheese, fresh basil, and red pepper flakes before serving.