Preheat your oven to 400°F and line a rimmed baking sheet with parchment paper.
Clean the portobello mushroom caps with a damp cloth, remove the stems, and use a spoon to gently scrape out the dark gills.
Finely dice the onion and mince the garlic.
Heat the olive oil in a skillet over medium heat and sauté the onion, garlic, and chopped spinach until the spinach is wilted and the onions are translucent.
In a medium mixing bowl, combine the ground turkey, egg, sautéed vegetables, dried oregano, sea salt, and black pepper.
Place the mushroom caps on the prepared baking sheet, gill-side up, and stuff them generously with the turkey mixture.
Crumble the feta cheese over the top and sprinkle with the parmesan cheese.
Bake for 25 to 30 minutes until the turkey is fully cooked through and the cheese topping is bubbly and lightly browned.