Whisk the egg yolk in a medium-sized bowl until lightly mixed. Add the onions, parsley, breadcrumbs, salt, pepper, nutmeg, and allspice and stir to combine.
Add the ground pork and ground beef to the egg mixture. Use your hands or a large spoon to thoroughly mix the egg mixture into the ground meat mixture.
Scoop the meatballs into 2-inch portions and use the palms of your hands to roll them into smooth balls. Set aside.
Heat oil in a 10-inch skillet or sauté pan over medium heat. Add the meatballs to the pan and cook, turning them every 2 minutes until browned on all sides, about 8 minutes total. Transfer the meatballs to a plate.
Melt butter in the same skillet. Add flour and stir to form a paste; cook, stirring frequently for 30 seconds.
Add beef broth and whisk until smooth; bring to a simmer. Return meatballs to the pan and simmer in the gravy, uncovered, until the meatballs are cooked through and the gravy has thickened, about 5-7 minutes.
Remove from heat and stir in sour cream. Taste the gravy and add additional salt, if needed. Enjoy the meatballs and gravy as-is or serve with pasta or mashed potatoes.
Notes
Meatball Size: Keep meatballs uniform for even cooking and make sure the meatballs are rolled into tight balls so they don't fall apart while cooking.
Avoid overcooking: Don't overcook the meatballs or they will become dry.
Gravy Texture: Adjust sauce thickness by controlling the flour and beef broth ratio.
Herbs: Fresh herbs like parsley add brightness; dried herbs are an acceptable substitute.