Preheat your oven to 400°F (200°C) and line a baking sheet with parchment paper.
Clean the portobello mushrooms with a damp cloth, remove the stems, and use a spoon to gently scrape out the dark gills.
Whisk together half of the olive oil and the balsamic vinegar, then brush the mixture onto both sides of the mushroom caps.
In a large skillet over medium heat, sauté the diced onion and minced garlic in the remaining olive oil until fragrant and translucent.
Add the ground turkey, oregano, sea salt, and black pepper to the skillet, breaking the meat apart with a spatula until fully browned.
Stir in the fresh chopped spinach and cook for 1-2 minutes until just wilted, then remove the skillet from the heat.
Place the mushroom caps on the prepared baking sheet, gill-side up, and fill each generously with the turkey and spinach mixture.
Roast in the oven for 15-20 minutes until the mushrooms are tender and the filling is golden brown.