In a bowl, mix turkey, beef, bread crumbs, onion, egg yolk, 1/4 teaspoon salt, and pepper.
Divide meat mixture into 4 equal portions. Pat each portion around a metal skewer (flat ones are easiest to handle) to form a log 1 inch thick and 7 to 8 inches long.
Lay skewers on a lightly oiled barbecue grill over a solid bed of hot coals or high heat on a gas grill (you can hold your hand at grill level only 2 to 3 seconds); close lid on gas grill. Rotating skewers to brown meat evenly, cook until kebabs are no longer pink in center (cut to test), 7 to 10 minutes. If desired, about 1 minute before meat is done, brush lightly with the sauce of your choice.