Gather all ingredients.

Pulse onion in a food processor until finely chopped, about 5 (1-second) pulses. Scoop onions onto the center of a towel, gather up the towel ends, and squeeze to remove liquid.

Combine onions, lamb, beef, garlic, oregano, cumin, marjoram, rosemary, thyme, pepper, and salt in a large bowl. Mix with your hands until well combined. Cover and refrigerate 1 to 2 hours to allow the flavors to blend.

Preheat the oven to 325 degrees F (165 degrees C). Evenly coat a 9x5-inch loaf pan with cooking spray.

Working in 2 batches if necessary, place meat mixture into a food processor and pulse until finely chopped and the texture feels tacky, about 1 minute.

Transfer mixture into prepared loaf pan, and pack down to make sure there are no air pockets. Line a medium roasting pan with a damp kitchen towel. Place the loaf pan in the center of the towel-lined roasting pan, and transfer it into the preheated oven. Carefully pour boiling water into the roasting pan until it comes halfway up the sides of the loaf pan.

Bake in the preheated oven until gyro meat is no longer pink in the center, about 1 hour to 1 hour 25 minutes. An instant-read thermometer inserted into the center should read at least 160 degrees F (70 degrees C).

Pour off any accumulated fat, and allow to cool slightly before slicing thinly and serving.