Preheat oven to 400°F (200°C) and line a baking sheet with parchment paper.
Clean the portobello mushrooms by removing the stems and gently scraping out the dark gills with a spoon.
Finely dice the mushroom stems and yellow onion, then mince the garlic cloves.
Heat the extra virgin olive oil in a large skillet over medium heat, then sauté the onion, garlic, and diced mushroom stems until softened.
Add the ground turkey to the skillet, breaking it apart with a spatula, and cook until browned and no longer pink.
Stir in the sea salt, black pepper, fresh thyme, and baby spinach, cooking just until the spinach has wilted.
Place the mushroom caps on the prepared baking sheet, gill-side up, and fill each generously with the turkey and spinach mixture.
Sprinkle the grated parmesan cheese over the top of each stuffed mushroom.
Bake for 15-20 minutes until the mushrooms are tender and the cheese is lightly golden.
Remove from the oven and drizzle each mushroom with the balsamic glaze before serving.