Lamb-Feta Burgers with Tzatziki

Tzatziki: Combine all ingredients in a small bowl. Burgers: In a large bowl, gently combine lamb, feta, onion, oregano, garlic paste, salt, and pepper; form into 4 (1-inch-thick) patties. Refrigerate burgers. Prepare an outdoor grill with a medium-hot fire (or heat a stovetop grill pan over medium heat). Lightly coat burgers and squash slices with olive oil cooking spray. Grill burgers 9 minutes, turning once or twice, for medium-rare doneness. Grill squash slices 2 minutes per side, until tender, and pita breads 1 to 2 minutes, until lightly toasted. To assemble, place lettuce and tomato slices on half of the pitas, then top with a burger, some tzatziki, and grilled squash slices. Top with remaining pita. Serve with extra tzatziki on the side.