Una cosa rara, ossia Bellezza ed onestà (A Rare Thing, or Beauty and Honesty) is an opera by the composer Vicente Martín y Soler. It takes the form of a dramma giocoso in two acts. The libretto, by Lorenzo Da Ponte, is based on the play La luna de la sierra by Luis Vélez de Guevara. The opera was first performed at the Burgtheater, Vienna, on 17 November 1786. It was a huge success and was shown 78 times. Mozart quotes a melody from this opera, "O quanto un sì bel giubilo", in the orchestral accompaniment to the trio "Già la mensa è preparata" [pl] of the finale of Don Giovanni.^[1]

The town mayor and the Spanish prince Don Giovanni try to seduce the virtuous Lilla, who is engaged to Lubino. The queen finds out and resolves the state of affairs so Lilla is able to marry her beloved.