Trim the very top and bottom off the leeks. Halve them and rinse out any grit. Finely slice them. Warm a pan for 1 min, then add 1 tbsp oil and the leeks. Season with salt and pepper. Cover, keep the heat low and sweat for 5 mins till the leeks are soft.
While the leeks sweat, peel and dice the potato. Peel and crush the garlic. Add them both to the pan. Crumble in the stock cube. Pour in 700ml boiling water (or use hot homemade stock). Turn up the heat. Cover, bring to the boil then turn the heat down and simmer for 20 mins till the potato is tender.
Trim any really thick stalks off the kale. Chop the leaves. When the potato is soft, add the kale to the pan. Simmer for 8 mins to wilt the kale, then ladle it into a blender and blitz (or use a hand-held blender in the pan).
Pare a few strips of the zest off the lemon. Squeeze a little juice into the soup. Taste it and add more salt, pepper or lemon juice if you think it needs it. Serve in warm bowls with the lemon zest on top.