Peel the potatoes and slice them thinly. Wash, trim and thinly slice the leek. Peel and trim the carrot, cut into small cubes.
Melt the butter in a saucepan, add onion and sauté until translucent. Add the other vegetables, sauté for a few minutes, then add the stock. Season to taste with marjoram, salt and pepper. Simmer for 15-20 minutes.
Dice the bacon and sauté in a dry skillet until slightly crispy. Add the bacon to the soup just before serving.