Cool and store for a make-ahead meal. Reheat covered, in a preheated 375ºF oven with a splash of water in the bottom of the dish. Once meatballs are hot, uncover and let them crisp up for 5 minutes.
For the Mash, fill a pot half way with cold water, peel and add potatoes as they are chopped or, if you peeled up the potatoes ahead place in a pot and fill with fresh water. (You can make the potatoes the night you serve or peel up the potatoes ahead of time and store them in fridge covered in water.) Add enough water to cover the potatoes then cover the pot and place over high heat to bring to a boil. Once water boils, remove the lid and salt the water and cook uncovered at low boil until tender to fork. Drain and return to hot pot. Mash the potatoes with sour cream and cream, a liberal dose of pepper, horseradish and chives, and salt to your taste. If you wish to make the potatoes ahead of service time and hold them for a couple of hours, place an inch of water in a large pot and bring to boil. Reduce heat to low and place a smaller pot filled with the potatoes into simmering water and cover that pot.
For the gravy, melt butter in saucepan over medium-high heat. Whisk in flour, bring to a bubble, whisk in stock and Worcestershire, season with black pepper and thicken. Temper egg yolk by lightly beating a ladle of gravy into it then whisk into sauce. Cool and store, and reheat over medium heat, adding a little more stock if necessary to loosen.
In shallow bowls, spoon in enough potatoes to gently mound up then arrange a few meatballs in the center of the mound, ladle gravy down over top and garnish with parsley.