Preheat your oven to 400°F (200°C) and line a rimmed baking sheet with parchment paper.
Clean the portobello mushroom caps with a damp cloth, remove the stems, and use a spoon to gently scrape out the dark gills.
Finely chop the mushroom stems and set them aside to use in the filling.
Heat the olive oil in a large skillet over medium heat, then add the diced red onion and minced garlic, sautéing until fragrant.
Add the ground turkey and chopped mushroom stems to the skillet, cooking until the meat is browned and cooked through.
Stir in the dried oregano, sea salt, black pepper, and fresh parsley, then remove the skillet from the heat.
Place the mushroom caps on the prepared baking sheet with the gill-side facing up and fill each cap generously with the turkey mixture.
Bake for 15 to 20 minutes until the mushrooms are tender and the stuffing is slightly golden on top.
Remove from the oven and drizzle the balsamic glaze over the mushrooms before serving hot.