In a large pot melt butter over medium heat. Add potatoes, onions, celery and parsley. Steam until potatoes are tender, do not fry, you do not want to brown the potatoes.
Put steamed mixutre into a large mixing bowl. Add eggs, a splash of milk (I eyeball the amount of milk, its just like making mashed potatoes, sometimes you need more and sometimes less) and breadcubes. Season to taste with salt and pepper. Beat with handheld mixture (or mash) till fluffy.