Melt the butter in a medium skillet over medium-high heat. Add the onion, season with salt and cook until golden brown, 4 to 6 minutes; set aside.
Combine 1/2 cup breadcrumbs and the milk in a bowl; set aside until the milk is absorbed, 1 to 2 minutes. Stir the sherry and mustard powder in a large bowl until dissolved, then beat in 1 egg. Add the soaked breadcrumbs, the browned onion, the pork, turkey, honey, allspice, 2 teaspoons salt, and pepper to taste. Gently mix with your hands until combined.
Dampen your hands; form the mixture into 36 small meatballs, about 1 tablespoon each. Put on a parchment paper-lined baking sheet, cover with plastic wrap and refrigerate at least 4 hours or overnight.
Put the remaining 1 cup breadcrumbs in a shallow dish. Whisk the remaining 2 eggs and 2 tablespoons water in a bowl. Dip each meatball in egg, letting the excess drip off, then roll in the breadcrumbs; return to the baking sheet.
Heat about 1 1/2 inches vegetable oil in a large saucepan over medium-high heat until a deep-fry thermometer registers 325 degrees F. Working in batches, fry the meatballs, gently stirring with a slotted spoon, until golden and cooked through, 3 to 4 minutes. Transfer to a paper towel-lined plate and season with salt. Let stand 10 minutes.
Fold the lingonberry preserves into the sour cream and top with the dill. Skewer each meatball with a pickled cucumber slice and serve with the lingonberry cream.