Peel and finely chop the onion. Grate the cheese on a medium grater or finely chop. Wash and dry the arugula. Wash, dry, and halve the cherry tomatoes.

Wash the eggs with warm water, dry and break - whites carefully (there must not be even a bit of yolk) pour into a clean and dry tall container such as a blender jar, and yolks into a bowl. Stir the yolks with a fork. Add a pinch of salt to the whites and beat with a mixer or a kitchen whisk until stiff peaks form.

Very gently combine the egg white foam with the stirred yolks. Mix thoroughly with gentle movements.

Heat the butter in a pan. After a while, add the onion. Fry for a few minutes, stirring occasionally.

Evenly spread the fried onion over the entire surface of the pan. Then transfer the egg mixture to the pan. Smooth from the top. Fry over low heat, covered. After about 3-5 minutes, flip to the other side. Be careful not to burn the omelet.

Immediately after flipping the omelet to the other side, evenly spread the ketchup and distribute the cheese. Wait until the cheese melts.

Transfer the ready omelet to a plate. Place arugula and cherry tomatoes on top. Eat immediately after preparation.