1. Start by gently cleaning the Portobello mushroom caps with a damp cloth or paper towel. Remove the stems, which can be chopped and added to the filling if desired. Brush the mushroom caps with a bit of olive oil on both sides, and place them gill-side up on a parchment-lined baking sheet, ready to be filled.

2. In a skillet, heat a tablespoon of olive oil over medium heat. Sauté the minced garlic until fragrant, then add the chopped spinach and cook until wilted. Stir in the diced cherry tomatoes, breadcrumbs, grated Parmesan, and Italian seasoning. Season with salt and pepper to taste, creating a flavorful and textured filling.

3. Spoon the prepared filling into each Portobello mushroom cap, pressing it in gently to ensure it’s evenly distributed. Top each stuffed mushroom with a generous sprinkle of shredded mozzarella cheese, ensuring the caps are fully covered.

4. Preheat your oven to 375°F (190°C). Bake the stuffed Portobello mushrooms for 18-20 minutes, or until the mushrooms are tender and the cheese on top is melted and bubbly.

5. Serve the hot, freshly baked stuffed Portobello mushrooms immediately, garnished with fresh basil or parsley, if desired.