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 chop the mushroom stems, onion, garlic, and spinach.
In a medium skillet, heat the olive oil over medium heat. Sauté the onion and chopped mushroom stems for 3-4 minutes until softened.
Add the garlic and spinach to the skillet, cooking just until the spinach is wilted, then remove from heat and let cool slightly.
In a large mixing bowl, combine the ground turkey, the whisked egg, the sautéed vegetable mixture, rosemary, thyme, sea salt, and black pepper.
Mix the turkey and herb mixture by hand until well combined, but do not overwork the meat.
Place the mushroom caps on the prepared baking sheet, stem-side up, and divide the turkey stuffing evenly between the two caps, mounding it slightly.
Bake for 25-30 minutes, or until the turkey reaches an internal temperature of 165°F.
During the last 5 minutes of baking, sprinkle the parmesan cheese over the tops of the mushrooms and return to the oven to melt.
Remove from the oven and let rest for 5 minutes before serving.