In a medium bowl mix all ingredients together until well combined. Refrigerate until burgers are ready to serve.
In a large bowl mix together lamb, salt and pepper, garlic, onion, Tzatziki, egg, sundried tomatoes, oregano, chili flakes. Split into 4 portions and shape into 1-inch thick burgers. Heat oil in a large pan over medium heat. Cook burgers for around 7 minutes on each side, or until burgers are cooked through. Serve on a bun topped with feta cheese, Tzatziki, baby spinach and a slice of green tomato.