Gergana by Bulgarian composer Georgi Atanasov. Audio only but an interesting rarity.

The libretto derives from a poem by Petko Slaveikov itself based on folk sayings, according to which the base of each bridge or large sanctuary needs to "enclose" the shadow of a loved woman. Legend has it, there are "enclosed" shadows in several bridges in various places in Bulgaria. There are stories of a noble Turkish ruler who had wanted the Bulgarian woman for his sanctuary, but acknowledged her love for another man. The opera combines this legend with the one of "enclosed shadows." Source: