Preheat your oven to 400°F (200°C) and line a baking sheet with parchment paper.
Clean the portobello mushrooms by wiping them with a damp cloth, then remove the stems and gently scrape out the dark gills with a spoon.
Finely dice the onion and mince the garlic cloves.
In a non-stick skillet over medium heat, add the olive oil followed by the onion and garlic, sautéing until translucent.
Add the ground turkey to the skillet, breaking it up with a wooden spoon, and cook until browned and fully cooked through.
Stir in the chopped parsley, thyme, sea salt, and black pepper, then remove the skillet from the heat.
Place the mushroom caps on the prepared baking sheet, gill-side up, and fill each cap generously with the turkey mixture.
Bake in the oven for 15-20 minutes until the mushrooms are tender and juicy.
Remove from the oven, drizzle with the balsamic glaze, and sprinkle with the crumbled feta cheese before serving.