Preheat dry cast iron skillet over medium heat on gas grill. Toast coriander and cumin seeds and whole peppercorns for 1-2 minutes in skillet. Cool; then grind together in spice mill. Reserve ¼ tsp of ground spices for raita and set aside.
Combine remaining ground spices in food processor with garlic, ginger, In a large mixing bowl, combine paste and ground chicken. Carefully distribute paste into chicken, but do not over handle. Use food scale to divide mixture into six equal portions and form into patties to fit the size of the buns. Wrap each patty in aluminum foil and refrigerate until you are ready to grill.
To prepare raita, sprinkle salt over sliced cucumbers and tomatoes and allow to drain on paper towels or in a colander for at least 15 minutes. Mix yogurt, lemon juice, garlic, mint and ground spices in bowl and set aside. Add the drained cucumbers and tomatoes to yogurt mixture. Cover and refrigerate until burger assembly.
Wash and dry lettuce. Select six leaves and chill until burger assembly.
Preheat gas grill to medium high.
Split buns and lightly brush insides with olive oil.
Remove patties from refrigerator and unwrap. Lightly brush grill with oil to prevent sticking. Grill patties for 5 minutes on one side and 5 minutes on second side or until juice runs clear when patty is pierced with fork.
During last few minutes of grilling, place buns with oiled side down and lightly toast on the grill.
Assemble burgers by placing lettuce leaf on bottom bun, followed by meat patty, raita and top bun. Serve burgers.