Heat a non-stick frying pan until very hot. Cut the salad onions in half lengthways and cook for about 2½ mins each side, cut-side first, until charred and tender. Transfer to a plate while you prepare the rest of the salad.
Cook the tomatoes, cut-side down, for 30 secs-1 min until just softened and caramelised, then set aside with the onions.
To make the dressing, put all the ingredients into a jar with some seasoning and shake well. When ready to serve, pile the watercress, onions and tomatoes onto a platter. Give the dressing a quick shake, then drizzle it all over the salad.