These saucy make-ahead meatballs make the perfect potluck dish.
Place the ground beef, breadcrumbs, onion, parsley, eggs, red pepper flakes and some salt and pepper in a large bowl and mix together thoroughly. Form the mixture into uniform-size balls.
Heat the oil and butter in a large pan. When hot, begin frying the meatballs in batches, setting them aside on a plate or baking sheet when brown. You may need to drain any excess fat from the pan during the frying process. Continue frying until all the meatballs are browned. Set aside.
Add the beef consomme and three-quarters of the can of beef broth to the meatball pan, reserving the remaining broth in a small bowl. Add the mustard and tomato paste to the pan, then stir and bring to the boil.
Add the cornstarch to the reserved beef broth and whisk so there are no lumps. As the sauce starts to boil, add the cornstarch mixture. When the sauce has thickened after a few minutes, return the meatballs to the pot and stir to make sure they are all coated in sauce. Continue to simmer until the meatballs are cooked through, about 45 minutes.