01 - Preheat oven to 375°F (190°C). Position a rack in the center.
02 - Heat a drizzle of olive oil in a medium skillet over medium heat. Add minced garlic and sauté for 30 seconds until fragrant. Stir in chopped spinach and cook until wilted, about 2 minutes. Remove from heat and allow to cool slightly.
03 - In a medium mixing bowl, combine sautéed spinach mixture, ricotta cheese, shredded mozzarella, and breadcrumbs. Season with salt and black pepper, stirring until thoroughly incorporated.
04 - Arrange portobello mushroom caps, gill side up, on a parchment-lined baking sheet. Evenly distribute the spinach filling among the mushroom caps, pressing the mixture gently for stability. Drizzle additional olive oil over the tops.
05 - Transfer baking sheet to the preheated oven. Bake for 20 to 25 minutes, or until the mushrooms are tender and the filling is lightly golden.
06 - Remove from oven and allow mushrooms to rest for 3 minutes before serving warm.