01 - Combine Greek yogurt, grated cucumber, minced garlic, dill, lemon juice, olive oil, salt, and pepper in a bowl. Mix thoroughly, cover, and refrigerate until ready to serve.
02 - In a large bowl, combine ground chicken, red onion, garlic, feta, parsley, dill, oregano, cumin, salt, and pepper. Mix gently just until incorporated; avoid overworking the meat.
03 - Divide the mixture into 4 equal portions and shape into patties, pressing lightly to form uniform thickness.
04 - Heat olive oil in a grill pan or skillet over medium heat. Cook patties for 5–6 minutes per side until golden brown and cooked through to an internal temperature of 165°F.
05 - Lightly toast the burger buns or pita breads until golden and warm.
06 - Layer lettuce on the bottom bun, add a chicken patty, spoon tzatziki generously over the top, and finish with tomato and onion slices. Close with the top bun.
07 - Serve immediately with extra tzatziki sauce on the side for dipping.