Preheat your oven to 400°F and line a baking sheet with parchment paper.
Clean the portobello mushrooms with a damp paper towel, remove the stems, and gently scrape out the black gills with a spoon.
In a medium skillet over medium-high heat, add the ground turkey and cook until browned, breaking it up with a spatula.
Stir in the minced garlic and chopped baby spinach, cooking for 2 minutes until the spinach is wilted.
Remove the skillet from heat and stir in the dried oregano, sea salt, black pepper, and 1 ounce of the crumbled feta cheese.
Brush the outside of the mushroom caps with olive oil and place them gill-side up on the prepared baking sheet.
Divide the turkey and herb mixture evenly between the two mushroom caps, pressing down slightly to pack the filling.
Sprinkle the remaining 0.5 ounce of feta cheese over the top of the stuffed mushrooms.
Bake in the oven for 15 to 20 minutes until the mushrooms are tender and the cheese is lightly golden.