1) Melt the butter in a frying pan.
2) Add the onions and sugar and cook until soft and golden brown.
3) Add the crushed garlic and mix thoroughly.
4) Pour in the water and the stock cubes and bring to the boil.
5) Add the potatoes and simmer on a low heat until they are soft (about an hour).
6) Blend with a suitable device and reheat for a further 10 minutes.
7) Season with salt & pepper to serve.