Place the mushrooms gill side up on a lined tray. Spray with olive oil and season to taste with salt and black pepper.
Place under the broiler for 3 to 4 minutes or until the mushrooms give up their liquid. You will smell cooked mushrooms. That's okay, as long as they don't burn. Drain the liquid.
Divide the pesto among the mushrooms and spread to coat each one well.
Divide the chicken among the mushrooms. Likewise the tomatoes.
Divide the feta cheese crumbles among the mushrooms. Sprinkle the mozzarella cheese over the mushroom caps as well.
Cook in the broiler or on a hot grill until the toppings are hot and the cheese is melted and browned in spots. Serve immediately.
Promptly store leftovers in an airtight container in the refrigerator, for up to 3 days.Nutritional values are approximate and based on 1 mushroom.Adaptations: feel free to vary the sauce and toppings according to your preferences. Use red sauce with pepperoni and sausage or BBQ sauce and pulled pork. You can do whatever you like!