Get Ready: Start by preheating your oven to 325°F (160°C). Grab a rimmed baking sheet and line it with parchment paper for easy cleanup.
Mix It Up: In a large bowl, combine your ground lamb and ground beef. Grate the onion directly into the bowl, then add the minced garlic, dried marjoram, cumin, rosemary, salt, and pepper.
Get Hands-On: Use your hands to thoroughly mix and squeeze the ingredients together. This step is key to getting that classic dense gyro texture!
Shape & Bake: Shape the mixture into a long, flat loaf, roughly 4 by 10 inches, and place it on your prepared baking sheet. Bake for 30 to 40 minutes, or until the center of the loaf reaches 160°F (71°C) with an instant-read thermometer.
Cool & Slice: Let the loaf cool completely. This is important for firm, easy-to-slice pieces! Once cooled, use a sharp knife to slice it into thin pieces, about 1/4 inch thick.
Quick Sear (Optional, but recommended!): If you like, heat a small amount of oil in a pan over medium-high heat. Quickly sear each gyro slice for about 1 minute per side until golden brown.
Serve Healthy: Enjoy your homemade gyro meat over a fresh salad or with cauliflower rice for a light, delicious, and healthy meal!