Wash and roughly chop the potatoes, leeks and garlic. Melt the butter in a large pan and add the chopped vegetables.
Stir until all the vegetables are coated with the butter, then put on the lid and and lower the heat. Leave to sweat for 15 minutes, stirring occasionally.
Add the stock and bring to the boil. Once boiling cover with the lid and simmer gently for 10 minutes. Add the watercress and allow to wilt (around 5 minutes)
Remove from the heat and blend using a hand-held blender. Season well.
If using creme fraiche, either add it at this point, or swirl a teaspoon of it on top of each soup serving.