Process the onion in a food processor for 10 to 15 seconds and turn out into the center of a tea towel. Gather up the ends of the towel and squeeze until almost all of the juice is removed. Discard juice.
Return the onion to the food processor and add the lamb, garlic, marjoram, rosemary, salt, and pepper and process until it is a fine paste, approximately 1 minute. Stop the processor as needed to scrape down sides of bowl.
Place the meat onto the rotisserie skewer by molding them into hamburger patty shape, then insert them into the skewer. Start by making smaller patties then increase their size as you move up the spit. Cook on high for 15 minutes. Decrease the heat to medium and continue to cook for another 20 to 30 minutes or until the internal temperature of the meat reaches 165 degrees F. Turn off the heat and allow to continue to spin for another 10 to 15 minutes or until the internal temperature reaches 175 degrees. Slice and serve on pita bread, chopped onion, tomatoes.