Heat half the oil in a heavy saucepan over a medium heat. Sear half the meat. When browned on all sides, remove the meat and repeat with the second batch. Remove the second batch of meat when browned. Add the remaining oil to the pan, then add the diced onion and when browned, return the meat to the pan. Add the water, and when boiling turn down the heat to a bare simmer, cover with a lid and simmer for 1 hour. Drain excess liquid, then add the reserved marinade, season with salt and pepper, oregano and bay leaves. Simmer, covered for a further 1 hour.