01 - Preheat the oven to 375°F (190°C).
02 - Remove the stems from the portobello mushrooms and gently brush both sides with olive oil. Season the mushrooms with salt and freshly ground black pepper.
03 - In a medium skillet, heat a drizzle of olive oil over medium heat. Add minced garlic and sauté until fragrant, about 30 seconds. Stir in chopped spinach and cook until wilted, about 2 minutes.
04 - Transfer the sautéed spinach and garlic to a bowl. Add ricotta, shredded mozzarella, and grated Parmesan cheeses. Mix until fully combined and season with salt and pepper.
05 - Spoon the cheese and spinach filling evenly into each mushroom cap, pressing gently to fill.
06 - Arrange stuffed mushrooms on a baking sheet. Bake in the preheated oven until the tops are golden and bubbling, about 20 minutes.