Preheat your oven to 400°F and line a baking sheet with parchment paper.
Clean the portobello caps by removing the stems and gently scraping out the dark gills with a spoon to create a hollow space for stuffing.
In a large skillet, heat the olive oil over medium heat and sauté the diced yellow onion and minced garlic until they become translucent and fragrant.
Add the ground turkey to the skillet, seasoning it with the sea salt, black pepper, and dried oregano.
Cook the turkey, breaking it up with a spatula, until it is completely browned and cooked through.
Stir in the chopped baby spinach and fresh parsley, cooking for 1-2 minutes until the greens have wilted, then remove the skillet from the heat.
Place the mushroom caps on the prepared baking sheet and divide the turkey mixture evenly between them, pressing down slightly.
Top each stuffed mushroom with the crumbled feta cheese.
Bake for 18-20 minutes until the mushrooms are tender and the cheese is slightly golden.
Drizzle the balsamic glaze over the mushrooms immediately before serving.