Heat the oil and sweat the bacon, onion and vegetables lightly in it. Dust with flour and pour some cold beef broth on top, allow to reduce slightly. Pour a little more beef broth again, then add porcini mushrooms and garlic and simmer for 10 minutes. Now add the potatoes and simmer until the potatoes are crispy and soft. Season with a dash of vinegar, marjoram, salt and pepper and thicken with liquid cream. Bring to the boil briefly and serve hot.