Preheat the oven to 350°f.
Using a handheld food processor or a knife, dice the onions very finely.
Add the meat, all the seasonings, onion, and garlic to a stand mixer fixed with the wire whisk attachment.
Whisk the gyro mixture until a paste-like consistency has formed.
Spray the inside of a loaf pan with cooking spray and spoon the meat into the loaf pan, firmly pressing and packing the meat into the pan.
Place the loaf pan in the oven and back for One Hour or until the internal temperature reads 190°f on an instant-read thermometer.
Remove the loaf from the pan, discard any remaining grease, and wrap the gyro meat in heavy-duty foil.
Place a cast iron skillet or any heavy dish on the loaf. Place a weight or large canned goods into the pan and allow the loaf to be pressed down for at least 30 minutes.
Once ready to serve, heat a cast Iron Skillet with a bit of olive oil over medium heat.
Slice 2-3 thin slices of meat per gyro from the loaf and place them in the skillet.
Lightly brown both sides, about 2 minutes a side.
Spread tzatziki sauce onto the center of a pita, lay the gyro meat on top of the sauce, then sliced tomato, red onion, and shredded lettuce.
Serve and Enjoy!