Lightly spray a non-stick frying pan with oil. Add the leek and garlic, and 1 tablespoon water to prevent sticking, then cook over low heat, stirring frequently, for 25 minutes, or until the leek turns golden. Add the cumin and cook for 2 minutes.
Put the potato in a large pan with the leek mixture and stock, bring to the boil, reduce the heat and simmer for 10-15 minutes, or until the potato is tender. Cool a little, then puree in batches, in a processor or blender, until smooth. Return to the pan and season.