Preheat your oven to 400°F and line a baking sheet with parchment paper for easy cleanup.
Gently wipe the portobello mushrooms clean with a damp cloth, remove the stems, and brush the outer caps with half of the olive oil.
In a medium skillet over medium-high heat, add the remaining olive oil and sauté the finely diced red onion and minced garlic for 2 minutes until fragrant.
Add the ground turkey to the skillet, breaking it up with a spatula, and cook until browned and no longer pink.
Fold in the fresh spinach and dried oregano, cooking just until the leaves have wilted, then season the mixture with sea salt and black pepper.
Place the mushroom caps gill-side up on the baking sheet and divide the turkey filling evenly among them, pressing down slightly to pack the stuffing.
Roast in the oven for 15 to 18 minutes until the mushrooms are tender and juicy.
Remove from the oven, sprinkle with crumbled feta cheese, and finish with a drizzle of tangy balsamic glaze before serving warm.