Remove any bits of skin and bone from the meat. Cut into serving-size slices, about 1/4 to 1/2 inch thick. Pound with a meat tenderizer or the edge of a heavy saucepan until 1/8 cm. thick. Trim.
Prepare a marinade of onion juice, olive oil, salt and pepper, and soak meat in the marinade overnight.
Spread over each piece of meat the lamb fat, and ground lamb mixed with an egg. Thread pieces of meat on a long skewer, starting with the larger pieces. Trim the chunk of meat on the skewer and add trimmings to the end of skewer. The tomato is put on the skewer whole at the end. The chunk of meat is broiled in the 'Doner Kebab' broiler, made specially for the purpose. The electric rotisseries they are selling today work well.
As the meat turns on the spit and is cooked, i t is sliced off the sides with a sharp knife.