Take the meat out of the refrigerator about an hour beforehand. Clean the lettuce and the small onion for the salad.
Clean the large onions and cut into slices with the slicer. Then fry the onion rings in butter and oil in the pan until crispy, remove from the pan and keep warm.
For the spaetzle dough, mix all the ingredients except for the salted water until the dough bubbles, then let it rest. Flatten the meat, cut into it and season with salt and sear it in oil and butter - approx. 3 minutes on each side. Reduce the heat, then wrap the meat in aluminum foil and cook in the oven at 90 ° C for 15-20 minutes.
Next, bring plenty of salted water to the boil for the spaetzle. Season the roast set with meat stock, salt and pepper, then bind with a sauce thickener and sour cream.
Scrape the spaetzle or press it through a spaetzle press. When they float on top, place in a bowl of warm water, then drain and place on a warm plate.
Dress the cleaned salad with the vinegar, oil and onions. Take the roast beef out of the oven and add to the spaetzle on the plate and serve with sauce.