1) Wash the potatoes and boil in their skins for 15–20 minutes in salted water. Drain and leave to cool slightly.
2) Chop the capers. Peel the shallots and finely dice, add to the hot stock. Mix together the capers, 3 tbsp. vinegar, salt, pepper and mustard. Peel the potatoes, slice and put into a bowl. Pour the stock mixture over the top, leave to infuse for 30 minutes.
3) Roast the pine nuts in a dry frying pan, leave to cool. Trim the broccoli, divide into florets, wash and leave to drain. Generously peel the broccoli stem and dice. Cook the florets and cubes in boiling, salted water for about 3 minutes until al dente. Drain, rinse in cold water and leave to drain. Trim the celery, remove any tough strings. Wash and cut into very thin slices.
4) Add the broccoli, celery and pine nuts to the potatoes. Pour the oil over the top and mix well. Remove the basil leaves, chop finely and fold in. Season to taste with salt, pepper and vinegar.