Preheat air fryer to 360°F.
In a large bowl, combine ground beef, ground pork, onion, milk, breadcrumbs, egg yolk, 1 teaspoon salt, 1/4 teaspoon black pepper, allspice and nutmeg. Mix well.
Roll meat mixture into golf ball sized meatballs.
Arrange meatballs inside air fryer.
Air fry for 6 minutes, turning half-way through.
Heat a medium skillet over medium high-heat.
Melt butter in skillet and whisk in flour.
Continue to mix until butter is fully disolved and a light beige roux forms.
Slowly whisk in broth, sour cream, remaining 1/2 teaspoon salt, and remaining 1/4 teaspoon black pepper.
Continue to cook, mixing constantly, until sauce is thickened. About 3 minutes.
Place meatballs inside a (6-inch) cake pan.
Pour sauce over cooked meatballs and carefully place cake pan inside air fryer.
Air fry an additional 3 minutes.
Serve with lingonberry sauce, if desired.