In a large bowl combine the gyro ingredients. Form the mixture into 12 small patties.
In a medium skillet, heat a few sprays of cooking spray over medium-high heat. Cook four patties for 2-3 minutes per side for medium rare. Repeat with remaining patties
In a medium-size bowl mix together the tzatziki sauce ingredients.
Warm the pita bread and place 3 patties in the bottom. Top with tzatziki sauce, diced onion, tomato and lettuce. Serve warm.