Preheat your oven to 400°F and line a baking sheet with parchment paper for easy cleanup.
Gently wipe the portobello mushrooms with a damp cloth, remove the stems, and scrape out the dark gills with a spoon.
In a medium skillet over medium heat, add the olive oil and sauté the minced garlic for 30 seconds until fragrant.
Add the ground turkey to the skillet, breaking it up with a spatula, and cook until browned and no longer pink.
Stir in the fresh spinach and cook for 1 to 2 minutes until completely wilted, then remove the skillet from the heat.
In a small bowl, combine the cooked turkey and spinach mixture with the Greek yogurt, dried oregano, sea salt, and black pepper.
Place the mushroom caps on the prepared baking sheet and divide the creamy turkey filling evenly among them.
Sprinkle the crumbled feta cheese over the top of each stuffed mushroom cap.
Bake for 15 to 20 minutes until the mushrooms are tender and the cheese develops a light golden crust.