Process the onion and garlic until very finely chopped, then combine them in a bowl with the lamb, beef, breadcrumbs, oregano, cumin, salt, Aleppo pepper, and black pepper. Work the mixture firmly with clean hands until it looks pale, sticky, and tightly bound rather than loose and crumbly.

Pack the meat firmly into a 9x5 inch loaf pan and bake at 325 degrees F for about 50-60 minutes, until the center reaches 165 degrees on a thermometer. Let the loaf rest for 20 minutes, then chill it for a few hours or overnight until it feels firm enough to slice cleanly.

Lift the chilled loaf from the pan and set aside the rendered drippings before cutting the meat into strips about 1/8 inch thick. A sharp knife should glide through the cold loaf and leave neat, narrow ribbons rather than ragged pieces.

Heat a griddle, cast iron skillet, or skillet over medium-high heat and lightly coat the surface with reserved drippings or a little oil. Sear the strips in a single layer for 1-2 minutes per side, until the edges sizzle and turn deep mahogany and crisp.

Fill each warmed pita with browned gyro meat, tomato, red onion, romaine, tzatziki, and feta if using. The pita should stay warm and pliable while the tzatziki cools the hot, crisp meat.