Cook the beans until done, approx. 30–40 minutes, then remove from the pan and rinse under cold water. Or, if you use beans from a jar, drain well and rinse. Prepare the dressing by combining all the ingredients and lastly, stirring in the oil in a trickle. Fold the beans into the dressing. Set aside and allow to marinate. Carefully remove the skin from the mackerel and then the meat from the bone. Separate the fish into flakes over a large bowl. Wash and halve the tomatoes and add them. If you use watercress, trim away the tough stems. Mix the lettuce with the fish and tomato. Lastly, fold in the beans and dressing and garnish with parsley.