Blend turkey, 3 tablespoons onion, garlic, salt, pepper, bread, and egg yolk and form into 1-inch meatballs. (I use my cookie dough scoop to form the meatballs).
Brown meatballs in olive oil in a non-stick skillet over medium heat; transfer to a bowl and set aside. Add remaining onion to the hot skillet, season with salt and pepper and cook over medium heat until onion is softened. Add mushrooms and sauté until the liquid the mushrooms give off has evaporated and mixture is fairly dry. Add broth, Worcestershire sauce and bring to a boil. Add cornstarch mixture and blend until sauce is glossy and thickened. Add meatballs with any accumulated juices, reduce heat, and simmer, covered, for 5 minutes. Turn off heat, stir in sour cream and dill, and adjust seasonings.