1In a large pot of boiling salted water, cook pasta according to package instructions; drain well.
In a large bowl, combine ground beef, bread crumbs, ketchup, mustard, Worcestershire and egg yolks; season with salt and pepper, to taste. Using a wooden spoon or clean hands, stir until well combined. Roll the mixture into 1 1/4-to-1 1/2-inch meatballs, forming about 20 meatballs.
2Melt butter in a large skillet over medium heat. Add meatballs, in batches, and cook until all sides are browned, about 4-5 minutes. Transfer to a paper towel-lined plate.
3To make the gravy, whisk together 1/2 cup beef broth and cornstarch in a small bowl; set aside.
Add onions to the skillet and cook until golden brown and translucent, about 2-3 minutes. Whisk in remaining 31/2 cups beef broth, Worcestershire and ketchup until well combined, about 1-2 minutes.
Bring to a boil; reduce heat and gradually whisk in cornstarch mixture. Stir in meatballs until cooked through and the sauce has thickened, about 3-4 minutes. Add more beef broth as needed until desired consistency is reached.