Peel, wash and cut the potatoes and carrots into small cubes. Clean and wash the leek and cut into fine rings. Peel and finely dice 2 onions and the garlic cloves.
Heat olive oil in a saucepan and sweat the onions in it. Add the garlic, potatoes and vegetables and continue to sweat. Pour the vegetable stock and season with salt, pepper and marjoram. Bring to the boil and cook for about 15 minutes until the potato cubes are soft.
Peel 2 large onions, cut into thin rings and in the meantime fry them slowly in a pan in a little olive oil.
Puree the potato soup with the blender. Stir in the oat cream and reheat the soup. Season to taste with salt and pepper and serve sprinkled with the fried onion rings.