Heat a serving platter in the warming oven. Place the corned beef into cold water and bring to the boil. Reduce heat to a simmer and cook for about 2 hours, turning occasionally. Place the vegetables in a big pan - first the potatoes - season - then the carrots - season - and then the onions, then cover with water. Cook until tender - approximately 1 hour. Strain the cooked vegetables and roughly mash them. Add the 1/2 cup of butter to make them rich and creamy. Drain the meat and slice it thinly. Spoon the vegetables (hutspot) on the heated serving platter and lay the sliced meat klapstuk over them.