Combine the ground meat, bread crumbs, egg, tzatziki sauce, garlic, onion flakes, mint, oregano, salt, and pepper in a large bowl. Mix thoroughly, then let the mixture sit for 10 minutes to allow the bread crumbs to absorb some of the moisture.
Shape the mixture into 1-inch meatballs, then roll them in flour.
Heat olive oil in a large skillet over medium-high heat. Place the meatballs in a single layer in the skillet and cook for 2-3 minutes on each side until browned. Reduce heat to medium and cook until the meatballs reach an internal temperature of 165°F, about 10 minutes.
Assemble the pitas by either cutting the top off the pita to create a pocket or, if using flatbread, laying it flat.
Spread about 2 tablespoons of tzatziki sauce inside the pita or on the flatbread. Add 4 meatballs, along with shredded lettuce, sliced cucumber, tomatoes, red onion, and a sprinkle of feta cheese.
Enjoy!