For 160 years, London’s National Gallery has been hunting down fragments of a rare altarpiece. The monumental panel, called The Pistoia Santa Trinity Altarpiece, was commissioned in 1455 and is the only surviving documented work by the greatest Renaissance artist you’ve probably never heard of: Francesco Pesellino. Wildly popular in his time, Pesellino painted for the Papal court in Rome and the Medici dynasty in Florence. In the summer of 1457, when plague swept through Tuscany, he died at 35 and never saw the Pistoia Trinity unveiled. In the 18th century the work was cut up and sold off. Pesellino’s name passed into obscurity and virtually his entire body of work was ascribed to other artists. But no more. “Pesellino: A Renaissance Master Revealed,” is now at London’s National Gallery. The first-ever exhibition on the artist, it was organized by Laura Llewellyn, the gallery’s curator of Italian Paintings before 1500. The show’s focal point is the altarpiece, now almost complete, and it is united with six other works by the artist. —Harry Seymour