Preheat your oven to 400°F and line a baking sheet with parchment paper.
Gently wipe the portabella mushrooms with a damp cloth to clean them, then remove the stems and scrape out the dark gills using a spoon.
In a large skillet over medium-high heat, add the olive oil and sauté the diced onion and minced garlic until fragrant and translucent.
Add the ground turkey to the skillet, breaking it up with a spatula, and cook until fully browned and no longer pink.
Stir in the chopped baby spinach, dried oregano, sea salt, and black pepper, 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.
Roast in the oven for 15 to 18 minutes until the mushrooms are tender and juicy.
Remove from the oven, drizzle each stuffed mushroom with balsamic glaze, and garnish with fresh parsley before serving.