Preheat your oven to 400°F and line a baking sheet with parchment paper.
Wipe the portobello mushrooms clean with a damp cloth, remove the stems, and gently scrape out the dark gills using a spoon.
Finely chop the mushroom stems, yellow onion, and garlic.
Heat the extra virgin olive oil in a skillet over medium heat, then sauté the onion, garlic, and chopped stems until softened.
Add the fresh spinach to the skillet and cook until wilted, then remove from heat and let cool slightly.
In a mixing bowl, combine the ground turkey, liquid egg whites, sautéed vegetable mixture, dried oregano, sea salt, and black pepper.
Place the mushroom caps on the prepared baking sheet and divide the turkey mixture evenly into each cap, pressing down firmly.
Sprinkle the nutritional yeast over the top of each stuffed mushroom.
Bake for 25 to 30 minutes until the turkey is cooked through and the tops are golden brown.
Remove from the oven and finish each mushroom with a drizzle of balsamic glaze before serving.