Peel and thinly slice the onions. Warm a large pan over a medium heat for 2 mins. Gloss with 1 tbsp oil. Add the onions and a good pinch of salt and pepper. Lower the heat. Sizzle for 5 mins, or till tender and a little golden.
While the onions fry, peel the potato, halve it and cut into 2cm slices at a diagonal. Add to the pot. Grate in the lemon zest and add a squeeze of the juice. Cook over low heat for 3-4 mins.
While the veg cook, peel and finely chop 2 of the garlic cloves. Scrub or peel the carrot. Coarsely grate it. Finely dice the celery. Stir them all into the pan. Sizzle over low heat for 3-4 mins.
Add the stock cube and 800ml boiling water. Simmer for 10-15 mins till the potato is just tender.
While the soup simmers, pick out any larger stalks from the watercress. Taste the soup. Season as needed. Pile the watercress on top, but don't stir it in. Take off the heat and set aside.
Peel the remaining 2 garlic cloves. Thinly slice them. Heat up a small frying pan. Gloss with 1 tbsp oil. Add the garlic slivers and a pinch of salt. Sizzle for 1-2 mins till just golden.
Ladle the soup into warm bowls. Spoon the crispy garlic and any oil over the top to serve.