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 use a spoon to gently scrape out the dark gills.
Finely dice the mushroom stems and set them aside.
Heat the olive oil in a large skillet over medium heat, then add the diced onion, garlic, and mushroom stems, sautéing until softened.
Add the ground turkey to the skillet, seasoning with sea salt, black pepper, and dried oregano.
Cook the turkey until browned and cooked through, breaking it into small crumbles with a spatula.
Stir in the fresh spinach and cook just until wilted, then remove the skillet from the heat.
Fold in the crumbled feta cheese and fresh parsley until well combined.
Place the mushroom caps on the prepared baking sheet, gill-side up, and fill them generously with the turkey mixture.
Bake for 20 minutes until the mushroom caps are tender and the filling is golden brown.