In a large bowl, combine ground beef, bread crumbs, egg yolks, nutmeg and cooked onion; season with salt and pepper, stir until well combined.
Heat 1 tablespoon olive oil in a large skillet over medium heat.
Add the onion, stirring frequently, until onions have become translucent, about 2-3 minutes.
Roll the mixture into golf ball sized meatballs, forming about 24 meatballs.
Add remaining 1 tablespoon olive oil to the skillet.
Add meatballs and cook until all sides are browned, about 4-5 minutes. Transfer to a plate lined with paper towel.
To make the gravy, melt butter in the skillet. Whisk in flour until lightly browned, about 1 minute.
Gradually whisk in beef broth, whisking constantly until slightly thickened about 1-2 minutes.
Stir in sour cream; season with salt and pepper, to taste.
Stir in meatballs, cook until thickened, about 8-10 minutes.