Preheat your oven to 325°F (165°C). Line a loaf pan or baking sheet with parchment paper and set aside.

In a large mixing bowl, combine ground beef, ground lamb, grated onion, minced garlic, oregano, cumin, coriander, paprika, black pepper, salt, cinnamon, and cayenne pepper. Mix thoroughly with your hands for 3-4 minutes until the mixture becomes sticky and cohesive.

Press the meat mixture firmly into the prepared loaf pan, eliminating any air pockets. Smooth the top and cover tightly with aluminum foil.

Bake for 60-75 minutes until the internal temperature reaches 165°F (74°C). The meat should be firm and browned on top.

While the meat cooks, prepare the tzatziki sauce. Grate the cucumber and squeeze out excess moisture. Combine cucumber, Greek yogurt, minced garlic, lemon juice, fresh dill, olive oil, salt, and pepper in a bowl. Mix well, cover, and refrigerate.

Remove the cooked gyros meat from the oven and let it rest for 10 minutes. Carefully drain any accumulated fat.

Once cooled slightly, slice the gyros meat as thinly as possible. Heat a large skillet over high heat and quickly sear the sliced meat until crispy and browned at the edges.

Warm the pita bread according to your preferred method.

To assemble, spread tzatziki sauce on warmed pita. Layer sliced gyros meat, tomatoes, onions, and lettuce. Add feta if desired, and squeeze fresh lemon juice over the filling.

Fold the pita around the filling, tucking in the bottom edge. Serve immediately while warm.