1. Put the onion, garlic (if using), leeks and ½ pint of chicken stock in a large heavy-based saucepan. Cover the pan and bring to the boil.
2. Continue boiling for 5-10 minutes, then uncover the pan, reduce the heat and simmer gently for about 20-30 minutes, until the onion is tender, golden and syrupy.
3. Add the potaoes to the cooked onion and leeks in the pan together with the remaining 1½ pints of hot chicken stock. Season to taste and simmer for 20 minutes.
4. Puree the soup in a blender or food processor and then return to the pan and stir in the skimmed milk and a grated nutmeg.
5. Reheat the soup gently and add a tsp of snipped chives. Alternatively, you can chill the soup and serve it ice-cold in the summer in the manner of vichyssoise.