Preheat oven to 400°F (200°C) and line a baking sheet with parchment paper.
Wipe the Portobello mushrooms clean with a damp cloth and carefully remove the stems; finely chop the stems to use in the filling.
Heat the olive oil in a large skillet over medium heat, then add the diced onion and minced garlic, sautéing until fragrant and translucent.
Add the ground turkey to the skillet, breaking it apart with a spatula, and cook until browned and no longer pink.
Stir in the chopped mushroom stems, spinach, dried oregano, sea salt, and black pepper, cooking until the spinach is fully wilted.
Place the mushroom caps on the prepared baking sheet, gill-side up, and fill each generously with the turkey and spinach mixture.
Bake for 15 to 20 minutes until the mushrooms are tender and the filling is slightly golden on top.
Remove from the oven, drizzle each mushroom with the balsamic glaze, and garnish with fresh parsley before serving.