Preheat your oven to 400°F (200°C) and line a baking sheet with parchment paper.
Clean the portobello mushrooms by wiping them with a damp cloth, then remove the stems and gently scrape out the dark gills with a spoon.
In a non-stick skillet over medium heat, add the olive oil and sauté the finely diced onion until translucent, about 3-4 minutes.
Add the minced garlic and ground turkey to the skillet, breaking the meat apart with a spatula, and cook until browned and no longer pink.
Stir in the chopped baby spinach, sea salt, and black pepper, cooking just until the spinach has wilted.
Remove the skillet from heat and stir in the fresh parsley and grated Parmesan cheese.
Place the mushroom caps on the prepared baking sheet, gill-side up, and divide the turkey mixture evenly between the two caps.
Bake in the oven for 15-20 minutes until the mushrooms are tender and the filling is golden.
Drizzle the balsamic glaze over the stuffed mushrooms before serving warm.