Preheat your oven to 400 degrees Fahrenheit and line a baking sheet with parchment paper.
Gently remove the stems from the portobello mushrooms and use a spoon to scrape out the dark gills, then brush the outsides with half of the olive oil.
Finely mince the garlic and roughly chop the baby spinach.
In a skillet over medium heat, add the remaining olive oil and sauté the ground turkey until browned, breaking it into small crumbles.
Stir in the minced garlic, sea salt, black pepper, and dried oregano, cooking for 1 minute until fragrant.
Add the chopped spinach to the skillet and stir until just wilted, then remove the pan from the heat.
Fold the crumbled feta cheese and fresh parsley into the turkey mixture until well combined.
Place the mushroom caps on the prepared baking sheet, gill-side up, and mound the turkey mixture generously into each cap.
Bake for 15 to 20 minutes until the mushrooms are tender and the filling is golden brown on top.
Serve immediately while the cheese is warm and the mushrooms are succulent.