Preheat your oven to 400°F and line a baking sheet with parchment paper.
Clean the portobello mushrooms with a damp cloth, remove the stems, and gently scrape out the dark gills with a spoon.
In a large skillet over medium heat, add the olive oil and ground turkey, breaking it up with a spatula.
Add the minced garlic, sea salt, black pepper, and dried oregano to the skillet, cooking until the turkey is browned and cooked through.
Stir in the fresh baby spinach and cook for 1-2 minutes until just wilted, then remove from heat.
Place the mushroom caps on the prepared baking sheet, stem-side up, and fill each generously with the turkey and spinach mixture.
Top each mushroom with 1 tablespoon of grated parmesan cheese and bake for 15-20 minutes until the mushrooms are tender.
While the mushrooms bake, place the balsamic vinegar in a small saucepan over low heat and simmer for 3-5 minutes until thickened into a glaze.
Remove the mushrooms from the oven and drizzle the warm balsamic glaze over the top before serving.