Preheat the oven to 400?F.
Pour 1/4 cup olive oil into a large mixing bowl. Add the mushroom caps and toss to coat.
Cook the sausages in a medium skillet over medium-high heat until browned, about 2 minutes. Add the onions, bell peppers, celery, and reserved minced mushroom stems and cook until softened, about 3 minutes. Add the garlic and cook until fragrant, about 30 seconds. Remove from the heat. Transfer the sausage-vegetable mixture to the bowl of a food processor. Add 2 tablespoons bread crumbs, 2 tablespoons Parmesan, 2 tablespoons parsley, the Essence, and the remaining teaspoon olive oil. Pulse until the stuffing comes together, about 30 seconds.
Fill each mushroom cap with a heaping teaspoon of stuffing. Place the mushrooms on a baking sheet. Combine the remaining 2 tablespoons bread crumbs and 2 tablespoons Parmesan in a small mixing bowl. Sprinkle over the mushroom caps. Bake the mushrooms until browned and tender, 15 to 18 minutes. Remove from the oven and let cool slightly. Garnish with the remaining 2 teaspoons parsley and serve warm or at room temperature.
Makes 2 dozen, 8 servings