Peel and finely dice the onion. Put minced meat and onion in a bowl. Add egg yolk, breadcrumbs and mustard, season with salt and pepper. Fold corn into the mixture and form 12 burgers.
Heat the oil in a pan. Fry the burgers at medium heat until golden brown and let them cool down lukewarm. In the meantime, wash and drain the rocket. Wash cherry tomatoes and cucumber and cut into slices.
Halve the burgers horizontally and spread 1/2 tsp ketchup on each. Spread some rocket on the lower halves and cover each with 1 cucumber slice and 2 tomato slices. Place the upper halves on top.
Arrange mini-burgers on plates.