Heat half of the olive oil in a large, high-walled skillet over medium heat. Add the diced onions and garlic. Cook until softened for about 5 minutes. Set aside to cool.
In a large bowl, mix breadcrumbs, Parmesan cheese, whisked egg, milk, cooled onions/garlic, salt, oregano, allspice, nutmeg, and pepper. Gently fold in the ground beef until just combined.
Shape the mixture into 1 1/2-inch meatballs and place them on a plate. Chill in the fridge for 15 minutes or up to overnight.
In a measuring cup, combine beef broth, chicken bouillon, Worcestershire sauce, Dijon mustard, and parsley. Set aside.
Heat the remaining olive oil in a skillet over medium-high heat. Brown the meatballs in batches, about 1 minute per side. Set aside.
Melt butter in the same skillet over medium heat. Stir in flour and cook for 2 minutes until it browns slightly.
Gradually add the beef broth mixture, stirring continuously. Bring to a boil, then simmer.
Place sour cream in a bowl, add some sauce, and mix well. Stir this mixture back into the sauce until combined.
Return the meatballs to the skillet along with any juices. Spoon the sauce over them. Simmer over low heat for 10-15 minutes until cooked through.
Garnish with fresh parsley and serve over mashed potatoes or egg noodles.
Notes
For a lighter version, you can use ground turkey instead of beef. No alcoholic drinks allowed, please substitute with a non-alcoholic option.