Preheat your oven to 400°F and line a rimmed baking sheet with parchment paper.
Clean the portobello mushrooms with a damp cloth, remove the stems, and finely chop the stems to save for the filling.
Heat the olive oil in a large skillet over medium heat, then add the diced onion, minced garlic, and chopped mushroom stems.
Sauté the aromatics for 3-4 minutes until softened, then add the ground turkey to the pan.
Cook the turkey, breaking it into small crumbles with a spatula, until it is completely browned and cooked through.
Stir in the chopped spinach, dried oregano, sea salt, and black pepper, cooking just until the spinach has wilted.
Remove the skillet from the heat and stir in the fresh parsley and lemon zest to brighten the flavors.
Place the mushroom caps gill-side up on the prepared baking sheet and divide the turkey mixture evenly between the two caps.
Bake for 15-20 minutes until the mushrooms are tender and the filling is golden on top.