Step 1: Preheat your oven to 325°F (165°C) and prepare a 9×5 inch loaf pan.
Step 2: Place the ground lamb, ground beef, salt, pepper, oregano, marjoram, and rosemary into a food processor.
Step 3: Pulse the meat mixture for 60-90 seconds until it becomes a fine, tacky paste with no visible chunks.
Step 4: Add the grated onion and minced garlic to the food processor and pulse a few more times until thoroughly combined.
Step 5: Transfer the gyro meat mixture into the loaf pan, pressing down firmly to eliminate air pockets.
Step 6: Place the loaf pan in a larger baking dish filled with about 1 inch of hot water (water bath) and bake for 60 minutes or until the internal temperature reaches 165°F.
Step 7: Remove from the oven, drain the excess fat, and place a second loaf pan on top of the meat with heavy weights inside.
Step 8: Let the gyro meat rest and compress for 30 minutes before slicing thin and searing in a pan for serving.