Step 1: Finely grate the onion and garlic. Place the grated onion and garlic in a fine-mesh sieve set over a bowl and press out as much liquid as possible. Discard the liquid.
Step 2: In a large mixing bowl, combine the ground lamb (or lamb/beef mixture), the pressed grated onion and garlic, dried oregano, dried marjoram, ground cumin, dried thyme, salt, and black pepper. Use your hands to thoroughly mix the ingredients until very well combined.
Step 3: Place the meat mixture into a food processor. Process on high speed for 1-2 minutes, scraping down the sides as needed, until the mixture forms a smooth, sticky paste.
Step 4: Tightly pack the processed meat mixture into a loaf pan (9x5 inch). Press down firmly to remove any air pockets. Cover with plastic wrap and refrigerate for at least 4 hours, or preferably overnight.
Step 5: Preheat your oven to 325°F (160°C). Place the chilled meat loaf onto a wire rack set over a baking sheet. Bake for 60-75 minutes, or until the internal temperature reaches 165°F (74°C).
Step 6: Once cooked, remove the loaf from the oven and let it rest for at least 15-20 minutes. This allows the juices to redistribute, ensuring tender and moist gyro meat.
Step 7: To serve, use a very sharp knife to thinly slice the gyro meat. For extra crispiness, quickly pan-fry the sliced meat in a hot skillet with a little olive oil for 1-2 minutes per side until lightly browned.