Preheat your oven to 400°F and line a rimmed baking sheet with parchment paper.
Clean the portobello mushroom caps with a damp paper towel to remove debris, then gently remove the stems and finely chop them.
Heat the extra virgin olive oil in a large skillet over medium heat.
Add the diced yellow onion, minced garlic, and chopped mushroom stems to the skillet, sautéing until softened and fragrant.
Add the ground turkey to the skillet, breaking it up with a wooden spoon, and cook until browned and fully cooked through.
Stir in the chopped baby spinach, dried oregano, sea salt, and black pepper, cooking just until the spinach has wilted.
Place the mushroom caps on the prepared baking sheet with the gill-side facing up.
Divide the turkey and spinach mixture evenly between the two mushroom caps, pressing down slightly to pack the filling.
Bake in the oven for 15 to 20 minutes until the mushrooms are tender and the filling is slightly golden on top.
Garnish with chopped fresh parsley and serve immediately.