Gareth Matheson, former chef at RAF Valley, where Prince William was stationed, cooks up his signature secret recipe Royal Welsh Lamb burger at the Royal Welsh Show, which was a Prince William’s favorite dish. The recipe has today been released to celebrate Prince Georges 2nd birthday. July 21 2015. See SWNS story SWBURGER: A former RAF chef has revealed the secret ingredients of Prince William’s favourite BURGER – bacon and egg. Gareth Matheson’s ‘Royal Welsh Lamb Burger’ was a hit with the Duke of Cambridge, despite including a combination of mint sauce and paprika. The mouth-watering burger also came with lean Welsh lamb mince and topped with the unusual Snowdonia cheese, Welsh-cured bacon and a fried egg. The 31-year-old chef would cook the burger for William when he worked in the officer’s mess at RAF Valley on Anglesey, north Wales.