Preheat your oven to 400°F (200°C) and line a baking sheet with parchment paper.
Clean the portobello mushrooms with a damp cloth, remove the stems, and use a spoon to gently scrape out the dark gills.
In a large skillet over medium heat, add the olive oil, finely diced yellow onion, and minced garlic, sautéing for 3 minutes until translucent.
Add the ground turkey to the skillet, breaking it apart with a spatula, and cook until fully browned and no pink remains.
Stir in the chopped fresh spinach, dried oregano, sea salt, and black pepper, cooking just until the spinach has wilted.
Place the mushroom caps on the prepared baking sheet with the cavity side facing up.
Divide the turkey and spinach mixture evenly between the two mushroom caps, pressing down slightly to pack the filling.
Bake in the center of the oven for 15 to 18 minutes until the mushrooms are tender and juicy.
Remove from the oven and drizzle each stuffed mushroom with the balsamic glaze before serving immediately.