Preheat the oven to 400 degrees F. Line baking sheet with parchment paper, set aside.
In a small bowl, combine milk and breadcrumbs, and allow to sit for 10 minutes.
In a large mixing bowl, combine beef, pork, parsley, allspice, nutmeg, garlic powder, salt, pepper, grated onion, and 2 eggs.
After 10 minutes add the breadcrumb mixture to a large mixing bowl. Mix well, but do not over-mix.
Using a two-tablespoon cookie scoop, form evenly sized meatballs. Arrange evenly on the prepared baking sheet. Place in the oven and cook for 20 minutes or until internal temperature reaches 165 degrees F.
While meatballs are cooking, prepare sauce. Heat a large saucepan over medium heat, and add butter, once the butter begins to melt, whisk in flour. Add in beef broth, and lemon juice, and mix well. Stir in sour cream, and add in salt and pepper.
The sauce will begin to thicken.
Once meatballs have cooked, add to toss coat the meatballs.
Serve warm.