Preheat your oven to 400°F and line a rimmed baking sheet with parchment paper.
Clean the portobello mushrooms with a damp cloth, remove the stems, and gently scrape out the dark gills using a spoon.
Finely chop the mushroom stems and sauté them in a skillet with the olive oil and minced garlic over medium heat until softened.
Add the fresh spinach to the skillet and cook just until wilted, then remove from heat and let cool for a few minutes.
In a medium mixing bowl, combine the ground turkey, egg, sautéed mushroom and spinach mixture, dried oregano, sea salt, and black pepper.
Place the mushroom caps on the prepared baking sheet, cavity side up, and divide the turkey mixture evenly between the two caps.
Sprinkle the crumbled feta cheese over the top of the turkey filling.
Bake for 25 to 30 minutes, or until the turkey is fully cooked through and the cheese is lightly golden.
Let the mushrooms rest for 5 minutes before serving to allow the juices to settle.