Nearly 80 years after Nazis murdered its author, an opera pulled from a dusty trunk in a San Francisco basement is being performed for the first time in Germany. The opera, “Grete Minde,” was written in the early 1930s by Eugen Engel, a Berlin textile merchant who tried unsuccessfully to get it to the stage as Hitler rose to power and work as a Jewish composer became impossible. Engel was apprehended by Nazis in Holland and killed in the Sobibor concentration camp in 1943. But before his death, Engel gave his daughter, Eva, and her son a trunk filled with...