Preheat your oven to 400°F (200°C) and line a baking sheet with parchment paper.
Clean the portobello mushroom caps by wiping them with a damp paper towel and removing the stems and gills with a spoon.
Heat the extra virgin olive oil in a large skillet over medium-high heat.
Add the diced yellow onion and minced garlic to the skillet, sautéing for 3 minutes until fragrant and translucent.
Add the ground turkey to the skillet, breaking it up with a spatula, and cook until browned and fully cooked through.
Stir in the fresh baby spinach, dried oregano, sea salt, and black pepper, cooking until the spinach is just wilted.
Place the mushroom caps on the prepared baking sheet, hollow side up, and divide the turkey mixture evenly between them.
Sprinkle the feta cheese over the top of the stuffed mushrooms.
Bake for 15 to 20 minutes, or until the mushrooms are tender and the cheese is slightly softened.
Remove from the oven, drizzle with the balsamic glaze, and garnish with freshly chopped parsley before serving.