Leek and Potato Soup 1 tablespoon of olive oil 1 medium sliced onion 225g potatoes cut into cubes 2 medium leeks. Slice thinly. 1.2 litres/2 pints vegetable stock 150ml/5fl oz double cream or crème fraîche salt and freshly ground black pepper for seasoning 1.Heat the oil in a pan adding onions, potatoes and leeks. Cook for 3-4 minutes until they begin to soften 2.Add the vegetable stock and bring to the boil. Simmer until the vegetables are soft. 3.Use a blender to whisk together so the mixtures is smooth. Put it back into another pan, pour in the cream or crème fraîche and heat it through.