In a bowl stir together, chuck, 4 tablespoons onion, garlic, salt, pepper, bread crumbs, and egg yolks. Form into meatballs about 1-inch in diameter.
In a heavy skillet heat oil over moderately high heat until hot but not smoking. Cook meatballs, shaking skillet, until well browned, about 4 minutes. Transfer meatballs with a slotted spoon to paper towels to drain.
To skillet add remaining onion and salt and pepper to taste and cook over moderate heat, stirring until onion is softened. Add mushrooms and cook, stirring until liquid is evaporated. Add Sherry and boil until almost completely reduced. Add broth and Worchestershire sauce and bring to a boil. Add beurre manie, a little at a time, whisking, and boil for 2 minutes. Add meatballs and simmer, covered, 5 minutes. Stir in dill. Serve with sour cream.
Recipe courtesy of Gourmet Magazine