Preheat your oven to 400°F (200°C) and line a baking sheet with parchment paper.
Clean the portobello mushrooms by wiping them with a damp cloth, then remove the stems and gently scrape out the dark gills with a spoon.
Place the mushroom caps on the prepared baking sheet, gill-side up, and lightly brush the edges with a small portion of the olive oil.
In a large skillet over medium-high heat, add the remaining olive oil and the ground turkey, breaking it up with a spatula until browned and cooked through.
Add the minced garlic and baby spinach to the skillet, sautéing for 2 minutes until the spinach is wilted and fragrant.
Stir in the dried oregano, sea salt, and black pepper, then remove the skillet from the heat.
Divide the turkey and spinach mixture evenly among the mushroom caps, pressing down slightly to pack the filling.
Sprinkle the crumbled feta cheese over the top of each stuffed mushroom cap.
Bake for 15-20 minutes until the mushrooms are tender and the feta is slightly softened and golden.
Garnish with fresh chopped parsley before serving warm.