Cook the potatoes with the skin firm to the bite. Peel the onion and chop finely.
Heat the beef broth and mix well with salt, pepper, vinegar, oil and mustard. Peel the warm potatoes, cut into 5 mm thin slices and place in the still warm marinade. Mix carefully. Seal the salad airtight and marinate in the refrigerator for at least 2-3 hours, stirring carefully for half an hour (during this time the potatoes soak up almost the entire marinade).
Remove the lettuce, mix with the onions and serve.