Wash the potatoes and cook in boiling water for about 20 minutes. Soak rolls in cold water. Peel and finely dice onion. Pluck the marjoram leaves from the stalks. Knead minced meat, egg, pressed bread roll, onion and marjoram leaves.
Season with salt and pepper. Form 4 large or 8 small meatballs from the mince mixture. Fry in 2 tablespoons of hot oil for about 15 minutes, turning. Dice the bacon and drain it in a pan without fat.
Add the lentils and pour on the stock. Leave to swell on low heat for about 10 minutes. Drain the potatoes, let them cool down a little, peel and slice them. Clean, wash and chop spring onions and salad.
Stir the remaining oil and vinegar into the lentils. Season with salt and pepper. Mix potatoes, spring onions, salad and lentil vinaigrette. Serve lukewarm with the meatballs.