01 - 
                Preheat oven to 375°F (190°C). Line a baking sheet with parchment paper. Brush both sides of the mushroom caps with olive oil and place them gill-side up on the prepared baking sheet.
              
              
              
                02 - 
                Bake the mushroom caps for about 10 minutes until they start to release water. This helps prevent a soggy final dish.
              
              
              
                03 - 
                While mushrooms are pre-baking, heat remaining olive oil in a large skillet over medium heat. Add garlic and onion, cooking until softened, about 3-4 minutes. Add spinach and cook until wilted, about 2-3 minutes more. Remove from heat and let cool slightly.
              
              
              
                04 - 
                In a bowl, combine the spinach mixture with ricotta cheese, Parmesan cheese, salt, pepper, and nutmeg if using. Mix well until thoroughly combined.
              
              
              
                05 - 
                Spoon the cheese and spinach mixture evenly into the pre-baked mushroom caps. Top each with a portion of the shredded mozzarella cheese.
              
              
              
                06 - 
                Return stuffed mushrooms to the oven and bake for 15-20 minutes more, until cheese is bubbly and golden brown.
              
              
              
                07 - 
                Garnish with additional grated Parmesan cheese before serving. Enjoy while warm!