Roméo et Juliette

Gounod's Roméo et Juliette isn't exactly a rarity, but it is no longer one of the stalwarts of the opera repertoire, a position which it certainly occupied during the Victorian era (there were 102 performances at the Théâtre-Lyrique in Paris in the first year of its life, followed by 291 performances at the Opéra-Comique in the next twenty).