Recently restored, “The Adoration of the Mystic Lamb,” by Hubert and Jan van Eyck, in Ghent’s St. Bavo’s Cathedral is considered a masterpiece of late medieval art. Painted between 1430 and 1432, the 12-panel altarpiece is notable for having been one of the first northern European works to move away from idealization to a psychologically richer style of human portraiture and an exacting observation of nature. In 1934, two of the panels were stolen.