In a bowl stir together well turkey, 2 tablespoons onion, garlic, salt, pepper, bread crumbs, and egg yolk and form into meatballs about 1 inch in diameter. In a heavy skillet heat oil over moderately high heat until hot but not smoking and cook meatballs, shaking skillet, until browned well, 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 mushrooms give off is evaporated. Add Sherry and boil until almost completely reduced. Add broth and Worcestershire sauce and bring to a boil. Add beurre manie, a little at a time, whisking, and boil 2 minutes. Add meatballs and simmer, covered, 5 minutes. Stir in dill. Serve meatball Stroganov with accompaniments. Serves 2. Gourmet October 1994