Step 1: Prepare the Aromatics: In a food processor, pulse the peeled and quartered yellow onion until it forms a very fine paste, almost liquid with no chunks. Transfer the onion paste to a fine-mesh sieve and press out as much liquid as possible; this is crucial for texture.
Step 2: Combine Meats and Seasonings: In a large bowl, combine the ground lamb, ground beef, pressed onion paste, minced garlic, dried oregano, dried marjoram, dried thyme, garlic powder, onion powder, salt, black pepper, and optional cayenne pepper.
Step 3: Mix Thoroughly: Using your hands, knead the meat mixture vigorously for at least 5-7 minutes. This develops the proteins, creating the dense, uniform, and slightly springy texture characteristic of gyro meat. The mixture should become very sticky and cohesive.
Step 4: Form the Loaf: Preheat your oven to 300°F (150°C). Line a standard 9x5 inch loaf pan with parchment paper, leaving an overhang. Firmly press the meat mixture into the pan, ensuring no air pockets. Press down hard to compact the meat.
Step 5: Bake to Perfection: Place the loaf pan in a larger baking dish and add about 1 inch of hot water to the larger dish to create a water bath. Bake for 60-75 minutes, or until an instant-read thermometer inserted into the center reads 165°F (74°C).
Step 6: Rest and Chill: Remove the loaf pan from the water bath and drain any accumulated fat. Place a heavy object (like another loaf pan filled with cans) on top of the cooked loaf to further compact it. Let it cool completely to room temperature, then refrigerate for at least 4 hours, or preferably overnight. Chilling is essential for firm slicing.
Step 7: Slice and Sauté: Once thoroughly chilled, remove the loaf. Use a very sharp knife or electric carving knife to slice the loaf as thinly as possible. In a large skillet over medium-high heat, add a tablespoon of olive oil. Cook the sliced gyro meat in a single layer for 2-3 minutes per side, until beautifully browned and slightly crispy.
Step 8: Serve and Enjoy: Serve your crispy, flavorful gyro meat in warm pita bread with homemade tzatziki, fresh chopped tomatoes, sliced red onion, and fresh parsley.