Preheat your oven to 325°F (160°C). Line a rimmed baking sheet with parchment paper for easy cleanup.
In a large mixing bowl, combine the ground lamb and ground beef. Grate the onion directly into the bowl, then add the minced garlic, marjoram, cumin, rosemary, salt, and pepper.
Using your hands, thoroughly mix and squeeze the ingredients together. This is key to getting the classic gyro meat texture, so really get in there and compact it!
Shape the meat mixture into a long, flat loaf, roughly 4 by 10 inches, and place it on the prepared baking sheet.
Bake for 30 to 40 minutes, or until an instant-read thermometer inserted into the center of the loaf registers 160°F (71°C).
Remove the loaf from the oven and let it cool completely. For the cleanest, thinnest slices, you can even refrigerate it for a while after it cools.
Once cooled, use a very sharp knife to slice the loaf into thin pieces, ideally no thicker than 1/4 inch (about 0.6 cm).
To get that perfect browned finish and to reheat, warm a small amount of your preferred cooking oil in a pan over medium-high heat. Quickly sear the gyro slices on each side for about 1 minute until lightly browned.
Serve your delicious homemade gyro meat over a fresh salad, with cauliflower rice, or in a low-carb wrap for a healthy and satisfying meal!