Preheat your oven to 400°F (200°C) and line a baking sheet with parchment paper.
Clean the portobello mushrooms with a damp cloth, remove the stems, and use a spoon to gently scrape out the dark gills.
Brush the mushroom caps with half of the olive oil and place them gill-side up on the baking sheet; roast for 10 minutes.
While mushrooms roast, heat the remaining olive oil in a skillet over medium heat.
Add the ground turkey to the skillet and cook until browned, breaking it into small crumbles with a wooden spoon.
Stir in the minced garlic, sea salt, black pepper, and dried oregano, cooking for 1 minute until aromatic.
Add the fresh spinach to the skillet and sauté until just wilted, then remove from heat.
Stir the crumbled feta cheese and chopped fresh parsley into the turkey mixture.
Remove the mushroom caps from the oven and carefully drain any excess liquid that has pooled inside them.
Divide the turkey and spinach mixture evenly between the two mushroom caps, pressing down slightly to pack the filling.
Return the stuffed mushrooms to the oven and bake for an additional 5-7 minutes until the mushrooms are tender and the filling is hot.