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 gently scrape out the dark gills with a spoon.
In a medium skillet over medium heat, add the olive oil along with the diced red onion and minced garlic, sautéing until translucent.
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 just until the spinach has wilted.
Place the mushroom caps on the prepared baking sheet, gill-side up, and divide the turkey mixture evenly between them.
Sprinkle the crumbled feta cheese over the top of the stuffed mushrooms.
Bake for 15 to 20 minutes until the mushrooms are tender and the cheese is slightly golden.
Remove from the oven, drizzle with the balsamic glaze, and garnish with fresh parsley before serving.