Mix the chopped onion, corn, and cheese into the mashed potatoes.

Set up 3 plates: one with flour, one with the beaten egg, and one with breadcrumbs.

Using large spoons, shape the mashed potato mixture into patties, then dip each one in flour, then egg, then breadcrumbs.

Heat oil in a skillet to cover the bottom. Cook the croquettes for about 5 minutes on each side, also cooking the edges.

Serve hot—delicious dipped in mustard.