Preheat the oven to 400 degrees F.
Using your hands, lightly rub each portobello mushroom with 2 teaspoons of olive oil.
Cook the sausage in a medium skillet until browned, about 4 minutes. Add the onion, bell pepper, celery, and mushroom stems and cook until softened, about 3 minutes. Add the garlic and cook until fragrant, about 30 seconds. Remove from heat.
Transfer sausage-vegetable mixture to the bowl of a food processor. Add, 1/4 cup of the bread crumbs, 1/4 cup of Parmesan, 1/4 cup of parsley, the Essence, and the remaining 2 teaspoons of olive oil. Pulse until mixture is finely chopped and comes together slightly, about 30 seconds. Transfer the mixture to a bowl, add the egg, and stir until well combined.
Divide the filling among the mushroom caps, about 1/2 cup for each mushroom. Place the filled mushrooms on a baking sheet. Combine remaining 1/4 cup of bread crumbs and 1/4 cup of Parmesan in a small mixing bowl. Divide the bread crumb mixture evenly among the tops of the mushrooms. Bake until golden brown on top and the mushrooms are tender, about 15 to 18 minutes. Remove from oven and let cool slightly. Garnish with remaining tablespoon of parsley, drizzle with olive oil and vinegar and serve warm or at room temperature.
Yield: 6 mushrooms