Heat the oven to 400°F and line a rimmed sheet pan with parchment. Soak panko in milk for 5 minutes.
Stir egg, squeezed spinach, feta, scallions, garlic, oregano, salt and pepper into the crumbs. Fold in turkey gently.
Divide into 24 equal portions and roll into balls about 1 1/2 inches across. Arrange on the pan and brush lightly with olive oil.
Bake for 18 to 22 minutes until several meatball centers reach 165°F.
Freeze after cooking: refrigerate meatballs in a shallow container within 2 hours and chill fully, then spread on a clean lined tray and freeze until firm. Transfer to a freezer bag, label and freeze for up to 2 months.
Reheat from frozen: place 4 meatballs in a small covered baking dish with 1/4 cup broth and bake at 350°F for 25 to 35 minutes until every center reaches 165°F.
Serve 4 meatballs per portion, spooning a little hot broth over them.