2.Heat oil in large frying pan, cook corn and onion, stirring, until onion softens. Cool.
3.Combine corn mixture, sifted flour, soda, and egg in medium bowl, season.
4.Heat oiled large frying pan, drop rounded tablespoons of mixture, in batches, into pan, cook about 1 minute each side.
5.Make avocado smash. Mash ingredients with fork in small bowl. Season to taste.
6.Serve corn cakes topped with smash and sour cream.
Corn mixture can be made a day ahead, make the corn cakes on the morning of the party. Prepare the avocado smash as close to serving as possible; cover the surface with plastic wrap.