Preheat your oven to 400°F (200°C) and line a rimmed baking sheet with parchment paper.
Gently clean the portobello mushrooms with a damp cloth, remove the stems, and use a spoon to carefully scrape out the dark gills.
Finely dice the red onion and mince the garlic clove.
Heat the extra virgin olive oil in a medium skillet over medium-high heat.
Add the red onion and garlic to the skillet, sautéing for 2-3 minutes until the onion is translucent and fragrant.
Add the ground turkey to the skillet, breaking it into small crumbles with a wooden spoon, and cook until it is no longer pink.
Stir in the fresh baby spinach and cook for approximately 1 minute until the leaves are just wilted.
Remove the skillet from the heat and stir in the sea salt, black pepper, lemon zest, and lemon juice until well combined.
Place the mushroom caps on the prepared baking sheet, gill-side up, and divide the turkey and spinach mixture evenly between the two caps.
Sprinkle the crumbled feta cheese over the top of each stuffed mushroom.
Bake for 15-18 minutes until the mushrooms are tender and the feta has softened and turned slightly golden.
Garnish with freshly chopped parsley and serve immediately.