Preheat Broiler Or Prepare Barbeque. Combine Ingredients Lightly But Thoroughly In Large Bowl. Shape Into 16 Thin Patties And Broil, Turning Once, Until Done As Desired. For Sauce: Combine First 4 Ingredients In Small Bowl. Add Garlic Powder, Salt And Pepper To Taste. To Assemble Sandwich, Place 1 Meat Patty In Each Pita Half And Top With Yogurt Sauce And Onion Slices.