Coarsely grate the cheese. Peel onion and chop finely. Pour corn into a sieve and drain well.
Heat the fat in a pan and fry the onion cubes for about 5 minutes, turning them until transparent. Whisk eggs, milk and cheese, except for a little bit for sprinkling.
Add the egg mixture to the pan and stir slowly until it sticks. Arrange on bread and sprinkle with cheese.