Preheat your oven to 400°F (200°C) and line a baking sheet with parchment paper.
Clean the portobello mushrooms with a damp cloth, remove the stems, and finely chop the stems to use for the filling.
Brush the outer mushroom caps with olive oil and place them gill-side up on the prepared baking sheet.
In a non-stick skillet over medium heat, sauté the onion, garlic, and chopped mushroom stems until they are soft and fragrant.
Add the ground turkey to the skillet and cook until fully browned and crumbled.
Stir in the fresh spinach and cook for 1 minute until just wilted, then remove the skillet from the heat.
Fold in the Greek yogurt, sea salt, black pepper, fresh thyme, and fresh parsley until the mixture is creamy and well combined.
Spoon the turkey filling into the mushroom caps, pressing down firmly, and top with a sprinkle of parmesan cheese.
Bake for 18 to 20 minutes until the mushrooms are tender and the cheese on top is golden brown.