Toss the grated apple with the lemon zest, juice, and the cinnamon and let sit a few minutes (to soften the apple).
Whisk the sour cream, 1/2 cup of the sugar and egg yolks to blend. Stir in the grated apple (including any juices). In a separate bowl, sift the flour, baking powder, cinnamon, nutmeg and salt. Stir this into the sour cream mixture until blended, then stir in the melted butter. Whip the egg whites until foamy then add 2 tablespoons of sugar and continue whipping until they hold a soft peak. Fold the whites into the batter and use right away.
Pour the oil into a tabletop deep fryer following manufacturer’s instructions (or into a large, deep pot so that it is only filled about 3 inches). Heat the oil to 350F (use a candy thermometer if cooking doughnuts in a pot). Have a baking tray lined with paper towel, with a cooling rack placed over it. Use a small ice cream scoop and carefully drop scoops of batter into the oil, making sure there is enough room between them to cook evenly. Cook the fritters for about 3 minutes on each side, then remove using a slotted spoon and let drain and cool for at least 10 minutes before dusting and enjoying.
The fritters can be enjoyed warm or at room temperature on the day they are cooked.