Peel the onion. Cut cucumber, onion and Bündnerfleisch into cubes of equal size and mix to a tartar. Season eggs with salt and pepper and whisk them. Stir in cheese, except for something to garnish.
Heat 1 teaspoon of oil in a coated pan (28 cm Ø). Add 1/4 of the egg mixture to the pan and stir with a spatula. Slide the evenly coagulating creamy mixture into the front edge of the pan.
Allow to set for a short time, then tip onto a preheated, greased baking tray and keep warm. Wipe the pan with kitchen paper and repeat the process 3 times. Arrange omelettes and tartar on plates. Garnish with rocket and cheese.