Preheat your oven to 400°F (200°C) and line a baking sheet with parchment paper.
Gently wipe the mushroom caps clean with a damp paper towel and remove the stems. Finely chop the stems and set them aside.
Finely dice the yellow onion and mince the garlic cloves.
Heat the olive oil in a large skillet over medium heat. Add the onion, garlic, and chopped mushroom stems, sautéing for 3-4 minutes until softened.
Add the ground turkey to the skillet, breaking it up with a spatula. Cook until browned and fully cooked through.
Stir in the fresh baby spinach, dried oregano, sea salt, and black pepper. Continue cooking just until the spinach has wilted, then remove from heat.
Place the portobello caps on the prepared baking sheet, gill-side up.
Divide the turkey and spinach mixture evenly between the two caps, pressing down firmly to pack the stuffing.
Roast in the oven for 15-20 minutes, or until the mushrooms are tender and juicy.
Remove from the oven and drizzle the balsamic glaze over the top of each stuffed mushroom before serving.