Preheat your oven to 400°F (200°C) and line a baking sheet with parchment paper.
Clean the portobello mushrooms by wiping the caps with a damp paper towel. Remove the stems and use a spoon to gently scrape out the dark gills, then lightly brush the outside of the caps with a tiny bit of olive oil.
Finely mince the garlic and roughly chop the fresh spinach.
Heat the olive oil in a small skillet over medium heat. Add the garlic and spinach, sautéing for 2 minutes until the spinach is wilted. Remove from heat and let cool slightly.
In a medium mixing bowl, combine the ground turkey, egg whites, crumbled feta cheese, sautéed spinach and garlic, fresh thyme, chopped parsley, sea salt, and black pepper.
Mix the turkey mixture thoroughly with a fork or clean hands until all ingredients are evenly distributed.
Place the mushroom caps on the prepared baking sheet, gill-side up. Divide the turkey mixture evenly between the two caps, pressing it down firmly into the center.
Bake for 20-25 minutes, or until the turkey is fully cooked through and the mushrooms are tender.
Remove from the oven and let rest for 3 minutes to allow the juices to settle.
Drizzle each stuffed mushroom with the balsamic glaze just before serving for a clean, gourmet finish.