Air Fryer: Preheat your air fryer to 300 degrees Fahrenheit. Stand the pork belly slices in foil vertically with foil wrapped halfway up the slices. Cook for 5-10 minutes on low heat. Then, raise heat to 400 degrees Fahrenheit and roast for another 10-15 minutes until the fat is caramelized and crispy.
Conventional Oven: Preheat your oven to 300 degrees Fahrenheit and layer the slices of pork belly on a foil-lined tray. Cook on low heat for 20 minutes to tenderize the meat, then increase heat to 390 degrees Fahrenheit and roast for another 10-15 minutes to caramelize and crisp up the fat.
While the pork belly is cooking, wash, trim, and thinly chop the romaine lettuce. Season lightly with olive oil and a little salt (the tzatziki is already well-seasoned).
Wash and chop the cucumber and mix it into the tzatziki.
Once the pork belly cools slightly, carefully slice it as thinly as you can.
To assemble the gyro sandwiches, open up the pita bread and stuff with pork belly, tzatziki and the lettuce salad.