Restaurateur Keith McNally is losing two key executive chefs, Lee Hanson and Riad Nasr. Over 16 years, they’ve cooked at Pastis, Balthazar, Schiller’s Liquor Bar and Minetta Tavern. Hanson and Nasr said in a statement yesterday: “It’s time for us to brave new challenges . . . We wish Keith and his entire team nothing but the best and continued success.” The split comes as McNally’s Balthazar in London has drawn mostly hostile reviews and his Pastis here needs to nail down a new lease to see it through building reconstruction.