Preheat your oven to 375°F (190°C). Line a baking sheet with parchment paper for easy cleanup.
Clean the mushrooms with a damp cloth. Remove the stems carefully and chop them finely. Set the caps aside and place them on the prepared baking sheet.
In a skillet, heat the olive oil over medium heat. Add the chopped mushroom stems and sauté for about 3-4 minutes until softened. Add the minced garlic and chopped sage, cooking for another minute until fragrant.
In a mixing bowl, combine the sautéed mixture with breadcrumbs, ricotta, half of the Parmesan cheese, lemon zest, salt, and pepper. Stir until all ingredients are well incorporated.
Generously fill each mushroom cap with the filling mixture, pressing down slightly to ensure they are packed well. Sprinkle the remaining Parmesan cheese on top of each stuffed mushroom.
Place the baking sheet in the preheated oven and bake for 20-25 minutes, or until the mushrooms are tender and the tops are golden and toasty.
Once out of the oven, let them cool for a few minutes. Garnish with freshly chopped parsley if desired. Serve warm as an appetizer or side dish.