Combine the breadcrumbs and heavy cream in a large bowl, stirring with a fork until all the crumbs are moistened. Set aside for at least 5 minutes to let crumbs absorb liquid.

Melt 1 tablespoon of the butter in a 12-inch straight sided sauté pan set over medium heat. Add the onion and a pinch of salt and sweat until the onions are soft and golden, 4 to 5 minutes. Remove from the heat and set aside.

In the bowl with the breadcrumbs, add the sautéed onion, ground beef, pork, egg, nutmeg, salt and pepper and mix well with your hands until completely combined.

With wet hands (to keep the mixture from sticking), shape the mixture into meatballs the size of a small golf ball (about 2 tablespoons each), and set them on a tray. You should have about 28-30 meatballs. (You can do this in advance and chill the balls until you are ready to cook them)

Melt the rest of the butter in the skillet over medium-high heat. Add the meatballs, in batches if necessary, and cook, turning frequently, for about 7 minutes until browned on all sides and cooked through. Transfer the meatballs to a plate and drain off all but 1 tablespoon of fat from the skillet.

Decrease the heat to low and add the flour to the skillet. Whisk until lightly browned, approximately 1 to 2 minutes. Return the skillet to the heat, whisk in the stock, cream, preserves, pickle juice, Worcestershire sauce, and mustard, and bring to a simmer. Season to taste with salt and pepper.

If you want to reheat the meatballs, add them to the sauce, reduce the heat to medium and simmer for about five minutes, until the the meatballs are heated through.