01 - In a medium bowl, combine Greek yogurt, grated cucumber, dill, garlic, lemon juice, olive oil, salt, and pepper. Stir well, cover, and refrigerate until ready to use.
02 - In a large bowl, mix ground chicken, breadcrumbs, feta, red onion, parsley, garlic, oregano, cumin, salt, and pepper until just combined. Do not overmix.
03 - Divide the mixture into 4 equal portions and shape into burger patties.
04 - Heat olive oil in a large skillet or grill pan over medium heat. Cook the patties for 5–6 minutes per side, or until golden brown and cooked through (internal temperature should reach 165°F).
05 - Place lettuce leaves on the bottom half of each toasted bun. Top with a chicken patty, tomato slices, red onion, and a generous spoonful of tzatziki sauce. Cover with the top half of the bun.
06 - Serve immediately with extra tzatziki on the side, if desired.