Grind the ground chuck and lamb cubes through the coarse plate of a meat grinder twice to mix thoroughly.

In a food processor, combine onion, garlic, marjoram, rosemary, salt, and pepper and pulse until finely ground.

Mix the ground meat and onion mixture together in a bowl.

Regrind the mixture through a fine plate.

Pack the mixture into a 3x7 inch loaf pan.

Wrap tightly with plastic wrap and refrigerate overnight.

Preheat an oven to 325 degrees F (165 degrees C).

Remove and discard the plastic wrap from the loaf pan.

Bake the gyro meat in the preheated oven until the internal temperature reaches 140 degrees F (60 degrees C), about 1 hour.

Drain away all of the liquid that has accumulated in the pan.

Remove the loaf from the pan.

Place the meat on a rack set above a baking dish to catch the drips.

Continue baking until the internal temperature reaches 165 degrees F (75 degrees C), 15 to 30 minutes more.

Remove from the oven and allow to stand 15 minutes before slicing.