Preheat your oven to 400°F and line a baking sheet with parchment paper.
Clean the portobello mushrooms with a damp cloth, remove the stems, and scrape out the gills with a spoon.
In a large skillet over medium heat, add the olive oil, diced onion, and minced garlic, sautéing until fragrant.
Add the ground turkey to the skillet, breaking it apart with a spatula, and cook until fully browned and no longer pink.
Stir in the chopped spinach, dried oregano, sea salt, and black pepper, cooking until the spinach is just wilted.
Place the mushroom caps on the baking sheet, gill-side up, and divide the turkey mixture evenly between them.
Sprinkle the crumbled feta cheese over the top of each stuffed mushroom.
Bake for 15 to 18 minutes until the mushrooms are tender and the cheese is slightly softened.
Drizzle the balsamic glaze over the mushrooms before serving.