Peel and cut the potatoes into quarter pieces and place in a medium size kettle. Cover the potatoes with water and bring to a boil. Boil for 15 minutes or until soft.
In a separate pan place the chopped celery and onion with ½ cup butter and saute until the mixture starts to become soft (5 minutes on medium-high heat).
Chop the bread into small cubes and place into celery and onion mixture with two tablespoons of butter. Saute until the bread gets covered with the butter mixture (about 3 minutes).
Drain the water out of the potatoes. Mash the potatoes and add 4 tablespoons of butter. Mix milk and egg together and add to the potato mixture. Add celery mixture, parsley and flour. Add pepper and salt to taste, and mix well.
Put the potatoes into a well-greased casserole dish. Place the remaining butter on top of the potatoes. Bake at 350 for 40 minutes, or until the top of the casserole is brown.
At times gram would put crumbled smoked bacon in to change it up a bit.