Whisk sour cream, buttermilk, sugar, egg and salt together. Sift together flour, baking soda and nutmeg and add to sour cream mixture, stirring just to blend. Let dough sit while heating oil.
Heat 2 inches (5 cm) of oil in a high-sided pot to 350 F (180 C), measured on a fryer or sugar thermometer. With a small ice cream scoop or with a tablespoon, drop spoonfuls of dough carefully into oil. Let cook until doughnuts are a deep brown on one side, about 4 minutes, before turning and cooking to the same color on the other side (often they turn themselves over when they are cooked on one side!). Remove doughnuts with a slotted spoon and drain on paper towel.
Let doughnuts cool 10 minutes, then sift icing sugar on top and serve, or toss in cinnamon sugar while warm.