Preheat oven to 350 degrees F (175 degrees C).

Spread rhubarb out into the bottom of a cake pan; sprinkle 1/4 cup white sugar over the top.

Beat brown sugar, sour cream, butter, egg, and vanilla extract together in a bowl with an electric mixer until smooth and creamy. Add flour and baking soda; mix until batter is just combined. Pour batter over rhubarb; top with 1 tablespoon white sugar and nutmeg.

Bake in the preheated oven until a toothpick inserted into the center of the cake comes out clean, about 1 hour.