Preheat your oven to 400°F and line a rimmed baking sheet with parchment paper.
Clean the mushroom caps with a damp paper towel to remove debris and gently pop out the stems.
Finely chop the mushroom stems to use for the filling and set the caps aside.
In a large skillet over medium heat, add the olive oil along with the diced onion, minced garlic, and chopped mushroom stems.
Sauté for 4-5 minutes until the vegetables are softened and aromatic.
Add the ground turkey to the skillet, breaking it up with a spatula, and cook until no longer pink.
Stir in the chopped spinach, dried sage, dried thyme, sea salt, and black pepper, cooking until the spinach is fully wilted.
Place the mushroom caps gill-side up on the prepared baking sheet and spoon the turkey mixture generously into each cap.
Sprinkle the tops with grated parmesan cheese and bake for 15-20 minutes until the mushrooms are tender.
Remove from the oven and garnish with freshly chopped parsley before serving hot.