Preheat the oven to 400F. Spray a baking sheet with oil. Gently remove the stems, scoop out the gills and spray the tops of the mushrooms with oil, season with 1/8 tsp salt and fresh pepper.
Heat a large nonstick saute pan over medium heat, add oil, onion, garlic and red pepper and season with 1/8 tsp salt. Cook until soft, 3 to 4 minutes. Add the baby spinach and saute until wilted, about 1 minute
In a medium bowl add the ricotta, parmesan cheese and egg, mix well. Add the cooked vegetables and basil and mix. Stuff the mushrooms with ricotta mixture and top each with 2 tbsp marinara, 2 tbsp mozzarella.
Bake in the oven for 20 to 25 minutes. Garnish with basil and enjoy!