Preheat your oven to 400°F (200°C) and line a baking sheet with parchment paper for easy cleanup.
Clean the portobello mushrooms using a damp paper towel, remove the stems, and gently scrape out the dark gills with a spoon to create space for the filling.
Finely chop the mushroom stems, red onion, and garlic cloves.
Heat 0.25 tablespoons of olive oil in a large skillet over medium heat and sauté the onion, garlic, and chopped stems until they are softened and aromatic.
Add the ground turkey to the skillet, seasoning it with the dried oregano, sea salt, and black pepper, and cook until the meat is fully browned and crumbled.
Stir in the fresh baby spinach and continue cooking for about one minute until the leaves are just wilted, then remove the skillet from the heat.
Brush the exterior of the mushroom caps with the remaining 0.25 tablespoons of olive oil and place them gill-side up on the prepared baking sheet.
Divide the turkey and spinach mixture evenly between the two mushroom caps, pressing down slightly to pack the filling.
Top each stuffed mushroom with the crumbled feta cheese and bake for 15 to 20 minutes until the mushrooms are tender and the cheese is slightly golden.
Garnish the finished dish with freshly chopped parsley and serve immediately while hot.