A FEW years ago, the Montreal restaurant Joe Beef made a version of KFC’s Double Down sandwich, using foie gras in place of the fried chicken breasts. This is what passes as a joke for chef-owners and contrarions David McMillan and Fred Morin. The dish became a media sensation, and they served 50 a night. They kept raising the price, eventually to C$55 ($41). People still bought it. “We had built the perfect lure to attract food writers,” says Mr. McMillan. He and Mr. Morin eventually grew tired of it, and took it off the menu.