Preheat your oven to 400°F and line a rimmed baking sheet with parchment paper for easy cleanup.
Clean the portobello mushrooms with a damp cloth, remove the stems, and use a spoon to gently scrape out the dark gills.
Finely chop the mushroom stems and set them aside to use in the turkey filling.
Heat the olive oil in a large skillet over medium heat and sauté the diced onion and chopped mushroom stems until they become soft and translucent.
Add the ground turkey to the skillet, breaking it into small crumbles with a wooden spoon, and cook until it is fully browned.
Stir in the minced garlic, fresh spinach, sea salt, black pepper, and dried oregano, cooking just until the spinach is wilted.
Turn off the heat and fold in the fresh parsley and lemon zest to brighten the flavors of the filling.
Place the mushroom caps on the prepared baking sheet and divide the turkey mixture evenly between them, pressing down slightly.
Top each mushroom with the parmesan cheese and bake for 15 to 20 minutes until the mushrooms are tender and the cheese is golden.