Add the American lamb, parsley, oregano, mint, salt, dill and pepper to a large mixing bowl, using your hands to gently mix. Form the mixture into 4 equal patties.
Heat a grill to medium-high heat. Place patties on the grate and grill for about 4 minutes on each side for medium-rare burgers. Remove to a wood cutting board to rest.
For serving:
Open each half of a pita and spread a spoonful of the tzatziki on the inside of the pocket then add a patty. In a small bowl stir together the feta and sun-dried tomatoes and divide the mixture among the pitas then top with red onion and spinach. Serve immediately.