Preheat your oven to 400°F (200°C) and line a baking sheet with parchment paper.
Clean the portobello mushroom caps with a damp paper towel and carefully remove the stems.
Place the mushroom caps on the prepared baking sheet, gill-side up.
In a large skillet over medium heat, add the olive oil and sauté the diced red onion and minced garlic until fragrant and translucent.
Add the ground turkey to the skillet, seasoning with the dried oregano, sea salt, and black pepper.
Cook the turkey, breaking it up with a spatula, until it is fully browned and cooked through.
Stir in the fresh spinach and cook for 1-2 minutes until it is just wilted, then remove the skillet from the heat.
Spoon the turkey and spinach mixture evenly into the hollowed-out mushroom caps.
Top each stuffed mushroom with the crumbled feta cheese.
Bake in the oven for 15-20 minutes until the mushrooms are tender and the cheese is slightly golden.
While the mushrooms are roasting, place the balsamic vinegar in a small saucepan over low heat and simmer until it reduces by half into a thick glaze.
Drizzle the warm balsamic glaze over the stuffed mushrooms before serving.