For the meatballs: Preheat the oven to 400 degrees F.
Heat a saute pan over medium to medium-high heat; melt the butter. Add the onion, celery and garlic; season with salt, pepper and thyme. Cook to tender, 7 to 8 minutes. Deglaze with the sherry or wine; cool.
Soak the bread in milk while the veggies cool.
Place the meat in mixing bowl; season with salt and pepper. Squeeze the liquid out of the bread and crumble into the bowl. Add the cooked vegetables, parsley, Worcestershire, mustard and eggs; combine to mix but do not overwork.
Place parchment paper on a baking sheet and roll the meat into 24 balls. Spray with cooking spray and roast about 15 to 18 minutes until lightly crispy and golden and cooked through. Cool and store for a make-ahead meal. Reheat, covered, in a preheated 375-degree F oven with a splash of water or they will stick in the bottom of the dish. Once meatballs are hot, uncover and crisp up for 5 minutes.
For the mash: Fill a pot halfway with cold water. Peel and add the potatoes as they are chopped or, if you peeled up the potatoes ahead, place in a pot and fill with fresh water. Add enough water to cover the potatoes, then cover the pot and place over high heat to bring to a boil. Once the water boils, remove the lid, salt the water and cook, uncovered, at a low boil until tender to fork. Drain and return to hot pot. Mash the potatoes with the sour cream and cream, a liberal dose of pepper, horseradish and chives; salt to your taste. If you wish to make the potatoes ahead of time and hold them for a couple of hours, place an inch of water in a large pot and bring to boil, then reduce heat to low and place a smaller pot, filled with the potatoes, into simmering water and cover that pot.
For the gravy: Melt the butter in saucepan over medium-high heat. Whisk in the flour and bring to a bubble; whisk in the stock and Worcestershire, season with pepper and thicken. Temper the egg yolk by lightly beating a ladle of gravy into it, then whisk into the sauce. Cool and store; reheat over medium heat, adding a little more stock, if necessary, to loosen.
Serve in shallow bowls. Spoon in enough potatoes to gently mound up in a bowl, then arrange a few meatballs in the center of the mound. Ladle gravy down over top and garnish with parsley.
You can make the potatoes the night you serve, or peel up the potatoes ahead of time and store them in the fridge, covered in water.