• Place 2 tbsp butter and 1 tbsp vegetable oil in the bottom of a large pot and heat to a medium temperature.
  
• Slice 400 g (3½ cups) leeks. Take off the root end and the very top of the leek, then cut the leeks into slices. Put the leek slices in a bowl of cold water and leave them for 10 minutes.

  Agitate the water to release any soil then lift the leek slices into a colander to drain. Don't pour the water into the colander because the soil settles at the bottom of the bowl and you don't want to pour it back over the leeks.
  
• Add the leeks and 400 g (1 ¾) potatoes and stir around for about 5 minutes so that the vegetables are covered in the oil and butter and the leeks have started to soften.
  
• Add stock made up from 4 stock cubes and 1½ litres (3 pints) of water.
  
• Bring to a boil, then simmer for 30-40 minutes until the vegetables are soft.
  
• Once cooked, cool slightly, then blend with a stick blender or ladle into a liquidiser jug blender for a smoother finish.
  
• Add 300 ml (1¼ cups) milk or a mixture of milk and cream and stir through the soup. Check the seasoning and add more salt and pepper if required. Reheat gently before serving.
  

For the garnish (optional)

• Heat 1 tbsp of butter and 1 tsp of vegetable oil in a frying pan to a medium-high heat.
  
• Add the 4 tbsp sliced leeks and fry until golden and crispy. Drain on kitchen paper.
  
• Serve the warm soup in bowls with a swirl from the 150 ml (¼ pint) double cream and garnish with the fried leeks.