1. To prepare sauce, combine first 4 ingredients; cover and chill.
2. To prepare burgers, combine onion and next 6 ingredients (through turkey) in a large bowl. Divide mixture into 8 equal portions, shaping each into a 1/3-inch-thick oval patty.
3. Heat a large grill pan over medium-high heat. Coat pan with cooking spray. Add patties to pan; cook 5 minutes on each side or until well-marked and done. Arrange 1 lettuce leaf, 1 patty, and about 1 1/2 teaspoons sauce in each pita half.