Preheat your oven to 400°F (200°C) and line a baking sheet with parchment paper.
Clean the portobello mushrooms with a damp cloth, remove the stems, and finely chop the stems to use in the filling.
In a skillet over medium heat, add the olive oil and sauté the ground turkey and minced garlic until the turkey is browned and cooked through.
Stir in the chopped mushroom stems and spinach, cooking for 2 minutes until the spinach is wilted.
In a medium bowl, combine the turkey mixture with the egg whites, grated parmesan cheese, fresh parsley, sea salt, and black pepper.
Place the mushroom caps on the prepared baking sheet, gill-side up, and fill each cap generously with the turkey and herb mixture.
Bake for 20 to 25 minutes until the mushrooms are tender and the stuffing is golden brown on top.
Remove from the oven and drizzle each mushroom with the balsamic glaze before serving.