Peel onion for the tartar. Cut cucumbers, onion and Bündner Fleisch into evenly small cubes. Mix everything and season with a little salt and pepper.
Use a peeler to remove some shavings from the cheese. Grate the rest finely. For the omelette, whisk the eggs. Season with about 1/2 tsp. salt and 1 pinch of pepper. Stir in grated cheese.
Heat the oil in a large coated pan (28 cm Ø). Pour in the egg mixture and let it falter briefly. Then fold two opposite sides towards the middle and let them fry to the end - the top of the omelette should still be soft but dry.
Meanwhile clean, wash and dry rocket. Cut the omelette into quarters and serve with the tartar. Sprinkle with rocket and cheese shavings.