Mix ground beef, ground pork, onion, bread crumbs, egg, Worcestershire sauce, cinnamon, nutmeg, parsley, salt, and pepper in a large bowl until evenly mixed; shape into 12 2-inch balls.
Heat vegetable oil in a large skillet over medium-high heat. Cook meatballs in hot oil until browned on all sides and somewhat firm, 5 to 7 minutes.
Remove browned meatballs with a slotted spoon to a slow cooker.
Stir cornstarch into drippings remaining in the skillet; add beef broth. Bring the broth mixture to a simmer, reduce heat to medium-low, and simmer until thickened, about 2 minutes. Stir sour cream into the broth mixture until smooth.
Add mushrooms and stir. Season the sauce with salt and pepper; pour over meatballs in the slow cooker.