Preheat your oven to 375°F.
Gently remove the stems from the portobello mushrooms and scrape out the gills with a spoon if desired, then brush lightly with olive oil and season with a pinch of salt and pepper.
Place the mushrooms on a baking sheet, cap side up, and bake for about 8-10 minutes to slightly soften them.
Meanwhile, heat the olive oil in a skillet over medium heat. Add the diced onion and sauté until translucent, about 3-4 minutes.
Add the minced garlic and cook for an additional 30 seconds until fragrant.
Stir in the lean ground turkey, breaking it up with a spatula. Cook until the turkey is browned and fully cooked, about 5-7 minutes.
Mix in the fresh spinach and dried oregano, cooking until the spinach wilts, about 1-2 minutes. Season with salt and pepper to taste.
Remove the mushrooms from the oven and spoon the turkey-spinach mixture generously into each mushroom cap.
Return the stuffed mushrooms to the oven and bake for an additional 5 minutes to meld the flavors.
Serve warm and enjoy your healthy, protein-packed meal.