Heat 1 tablespoon of the oil in a pan and cook the onion for 5 minutes until soft. Add the garlic and half of the watercress and cook for a further 2 minutes.
Put the peppers and chickpeas in a food processor with the onion mixture, parsley and breadcrumbs. Whizz until combined. The mixture will be quite wet. Carefully shape into 4 patties and chill in the fridge for 10 minutes to firm up.
Heat the remaining oil in the pan and cook the burgers for 4-5 minutes each side, until golden.
Put each burger onto the bottom half of a boule, top with tomato slices and 1 tablespoon of the soured cream and chive dip. Put the remaining bread on top, garnish with the remaining watercress and serve.