Preheat your oven to 400°F and line a rimmed baking sheet with parchment paper for easy cleanup.
Clean the portobello mushrooms with a damp paper towel, remove the stems, and gently scrape out the dark gills using a spoon.
Heat the olive oil in a large skillet over medium heat and sauté the diced yellow onion and minced garlic until fragrant and translucent.
Add the ground turkey to the skillet, breaking it into small crumbles with a spatula, and cook until no longer pink.
Stir in the chopped spinach and dried oregano, cooking just until the greens have wilted, then remove from heat.
Allow the turkey mixture to cool slightly before stirring in the beaten egg and parmesan cheese to bind the filling together.
Season the mixture with sea salt and black pepper, ensuring the flavors are well-balanced.
Place the mushroom caps on the prepared baking sheet and divide the turkey stuffing evenly between them, pressing down slightly.
Bake for 20 minutes until the mushrooms are tender and the protein-rich filling is golden and set.
Transfer the mushrooms to a plate and finish with a decorative drizzle of balsamic glaze before serving warm.