Preheat your oven to 350°F (175°C).

Gently clean the portobello mushrooms and remove the stems and dark gills.

In a large skillet over medium heat, sauté minced garlic until fragrant, then add diced chicken, Italian seasoning, and red pepper flakes. Cook for 5-7 minutes until the chicken is cooked through.

Reduce heat to low, stir in marinara sauce and half of the mozzarella cheese, and mix thoroughly for 2-3 minutes.

Stuff each mushroom cap with the chicken mixture, top with remaining mozzarella and Parmesan cheese.

Bake for 25-30 minutes until mushrooms are tender and cheese is bubbly and golden brown.

Let cool for about 5 minutes before serving.