Swedish Meatballs

Name: Swedish Meatballs
Ingredients: 1 tablespoon unsalted butter; 1/2 medium onion, finely chopped; Kosher salt; 1 1/2 cups breadcrumbs; 1/3 cup milk; 2 teaspoons dry sherry; 1/2 teaspoon mustard powder; 3 large eggs; 1/2 pound ground pork; 1/2 pound ground turkey; 1 tablespoon honey; 1/2 teaspoon ground allspice; Freshly ground pepper; Vegetable oil, for frying; 3 tablespoons lingonberry or cranberry preserves; 1/2 cup sour cream; Chopped fresh dill, for topping; Fresh pickled cucumber slices, for serving