Preheat your oven to 400°F (200°C) and line a baking sheet with parchment paper.
Clean the portobello mushroom caps with a damp cloth, remove the stems, and finely chop the stems to use in the filling.
Heat the olive oil in a large skillet over medium heat, then add the diced onion, minced garlic, and chopped mushroom stems, sautéing until softened.
Add the ground turkey and dried oregano to the skillet, breaking the meat apart with a spatula and cooking until browned and fully cooked through.
Stir in the baby spinach and cook for 1-2 minutes until just wilted, then season the mixture with sea salt and black pepper.
Place the mushroom caps on the prepared baking sheet, gills facing up, and spoon the turkey and spinach mixture generously into each cap.
Sprinkle the parmesan cheese over the top of the stuffed mushrooms and bake for 15-18 minutes until the mushrooms are tender and the cheese is golden.
Remove from the oven, drizzle each mushroom with the balsamic glaze, and garnish with fresh chopped parsley before serving.