Peel and finely slice the garlic clove. Heat a pan. When warm, pour in 1 tbsp olive oil. Add in the sliced garlic with a pinch of salt. Fry for 3-4 mins till the garlic slices are golden and crispy. Remove from the pan with a slotted spoon onto kitchen paper.
Peel and finely chop the onion. Peel and finely dice the potato. Place the pan back on the heat. Add the onion and fry for 5 mins in the garlicky oil. Add the potato and cook for 3 mins. Crumble in the stock cube. Pour in 700ml boiling water. Cook for 10 mins.
Drain and rinse the cannellini beans. Add them to the simmering pan. Pick the leaves from half the watercress and set aside. Roughly chop the rest. Add all the chopped watercress to the pan. Add in the zest from the lemon and half the juice.
Blitz the soup till smooth using a stick blender or transfer to a food processor. Put the soup back on a low heat. Swirl through the reserved leaves. Taste and seaosn the soup, adding more lemon juice if necessary. Serve in warm bowls with the crispy garlic scattered on top and a drizzle of oil.