In a small bowl, soak bread in milk. Tear with hands until it resembles a soft paste.
In a large bowl, combine ground beef, ground pork, soaked bread, onions, eggs, salt, and allspice. Gently mix until combined.
Using a small scoop, portion about 1 tablespoon of meat mixture and using roll between palms of hands into a 1-inch diameter ball.
Arrange meatballs in a single layer on a parchment-lined baking sheet and freeze for about 20 to 30 minutes or until firm.
In a skillet over medium heat, the butter until melted. Add meatballs in a single layer and cook, gently turning on sides as needed, until lightly browned but not cooked through. Remove from pan and drain on paper towels. Keep warm.
In the pan over medium heat, sprinkle in the flour over the pan drippings (add more butter if needed to equal 6 tablespoons) and whisk until smooth and blended. Continue to cook, stirring regularly, until roux turns lightly golden.
Add sherry, whisking regularly, until smooth.
Slowly add beef stock, whisk until smooth, and continue to cook until sauce begins to thicken.
Add meatballs in a single layer. Lower heat, cover, and continue to cook for about 8 to 10 minutes or until meatballs are fully cooked.
Add sour cream and Dijon mustard and stir until well-dispersed. Season with pepper to taste.
Stir in the jelly or serve on the side. Serve hot with boiled or mashed potatoes or over egg noodles.