Preheat the oven to 400°F. Lightly oil a large sheet pan.

Toast the bread until just brown, then soak it in water for about 5 minutes. Discard the water and crumble the bread into a large mixing bowl.

In a skillet, heat the olive oil over medium heat, add the spinach, and briefly cook until wilted. Let cool, then add to the mixing bowl.

In the mixing bowl, add ground turkey, grated onion, minced garlic, crumbled feta, beaten egg, chopped parsley, mint, wilted spinach, and season with salt and pepper. Mix well.

Form the mixture into golf ball-sized meatballs and arrange on the prepared sheet pan. Bake for about 30 minutes, until fully cooked through.

Transfer the meatballs to the top rack and broil for a couple of minutes until the tops are browned, keeping a close eye to avoid burning.