Preheat your oven to 400°F and line a baking sheet with parchment paper.
Clean the portobello mushrooms by removing the stems and gently scraping out the dark gills with a spoon.
In a skillet over medium heat, add half of the olive oil and brown the ground turkey along with the minced garlic, oregano, sea salt, and black pepper.
Once the turkey is cooked through, stir in the fresh spinach until it is just wilted, then remove the skillet from the heat and fold in the crumbled feta cheese.
Brush the outside of the mushroom caps with the remaining olive oil and place them gill-side up on the prepared baking sheet.
Spoon the turkey and spinach mixture into the mushroom caps, pressing down slightly to pack the filling.
Bake for 15 to 20 minutes until the mushrooms are tender and the filling is slightly golden on top.
While the mushrooms bake, add the balsamic vinegar to a small saucepan over low heat and simmer until it reduces by half into a thick glaze.
Remove the mushrooms from the oven and drizzle the warm balsamic glaze over the top before serving.