Darina Allen's winter leek & potato soup

INGREDIENTS

• 50g butter
• 450g potatoes , peeled and cut into 1cm pieces (try Golden Wonders or Kerr Pinks)
• 1 small onion , cut the same size as the potatoes
• 450g white parts of leeks , sliced (save the green tops for another soup or stock)
• 850ml-1.2litres/1.5-2pts light chicken or vegetable stock
• 142ml carton whipping cream
• 125ml full-fat milk
• the white part of 1 leek
• a small knob of butter
• finely chopped chives