Preheat your oven to 400°F (200°C) and line a baking sheet with parchment paper.
Clean the portobello mushrooms with a damp cloth, remove the stems, and finely dice the stems to be used in the filling.
Place the mushroom caps on the prepared baking sheet, gill-side up, and lightly brush the edges with a small amount of the olive oil.
In a large skillet over medium heat, add the remaining olive oil and sauté the ground turkey, diced mushroom stems, and red onion until the turkey is browned and cooked through.
Add the minced garlic and chopped spinach to the skillet, stirring for about 2 minutes until the spinach is completely wilted and fragrant.
Remove the skillet from the heat and stir in the sea salt, black pepper, lemon zest, lemon juice, and fresh parsley.
Spoon the turkey and herb mixture evenly into the mushroom caps, pressing down slightly to ensure they are well-packed.
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 feta cheese is slightly golden and warm.