Prepare twelve poached eggs, drain them on a cloth and remove any remaining strands of egg white. Wash 200 grams of watercress and drain it carefully. Take six slices of bread, remove crusts, divide each slice in half and toast. Coat the slices with butter mixed with a little mustard, place them on a round dish topped with a poached egg. Surround with watercress seasoned with salt and a light dressing made from oil, a little vinegar, salt and pepper. Serve well chilled.