Imagine spending 30 years thinking about Offenbach: studying his work, researching his music, and seeking out his lost manuscripts. This has been the remarkable, lifelong focus of the French musicologist and conductor Jean-Christophe Keck. His tenacious passion was resoundingly justified on Sunday night, when we were treated to a single, live, concert performance of Fantasio at Southbank Centre, the electrifying culmination of a series of recording sessions by Opera Rara which will ensure this neglected opéra comique remains hidden no longer. The sooner it enters the regular repertoire, the better: many more audiences deserve to be enchanted by this wily, tender philosopher-student lover and his endearing Princess.

Fantasio, musically, is ravishing. Sir Mark Elder conjures an incredibly clear, bright, sharply accurate sound from the Orchestra of the Age of Enlightenment, who play with lyrical freshness and absolute precision: it is easily one of the most special things I have heard this year. The overture is full of beautiful, crisp, lucid, fairytale sounds, creating well-rounded harmonies. In the pre-performance talk, Elder suggested we listen out particularly for Offenbach’s innovative use of the harp and the clarinets; alongside many other treats, this tip repays close attention. The opera is delightfully paced and delicately balanced throughout: the sound is flowing, elegant, and full of narrative romance and drama, with contrasting light and shade. The three acts are each unique in tone, but work brilliantly together, making for an evening peppered with comedy, suspense, disguise and finally fulfilment. Without ever being saccharine or kitsch, this music is genuinely charming; within the first few scenes, I found myself already thinking it will make a perfect Glyndebourne (or any other festival) opera.

The singing was excellent, most particularly a stellar performance from Brenda Rae as the young Princess Elsbeth. It is not always easy to bring an opera to life in a concert performance, but Rae’s magnetic stage presence, and total immersion in her character, ensured Elsbeth is very real at all times. Rae apparently sang the role at short notice, but her performance was so relaxed, assured and joyous that we simply could not have guessed she came in at the last minute. She also has the happy gift of being able to remain in character and look entirely natural while others are singing their parts: an especial pleasure. Her liquid soprano balances beautifully with Sarah Connolly’s gentle mezzo as Fantasio, bringing balance and contrast to their breathtaking duets. The Opera Rara Chorus create a soft, magical backdrop. Russell Braun and Robert Murray are hilarious as the sharp-suited Italian Prince of Mantua and his overenthusiastic, fawning sidekick. Brindley Sherratt is a majestic and paternal King. Victoria Simmonds is excellent as Flamel. Neal Davies (Sparck), Aled Hall (Facio) and Gavan Ring (Hartmann) are raucous but sonorous students – obsessed with drinking, with one of my favourite lines from the libretto rousing their party spirit: “Come on, Sparck! Let’s get our teeth into the moon!” Stealing the show altogether were two fine (and hilarious) cameos by Sir Mark Elder himself, who joined in the performance while conducting, in two small parts: not only a maestro but also a virtuoso comic actor!

The libretto is a lyrical delight, either in French on stage or in Jonathan Burton’s well-made surtitles (in which Princess Elsbeth is “as wayward as a wagtail”). Thanks to the efforts of Nicole Tibbels, the French coach, and Agathe Mélinand as dramaturg, the French sounds authentic and the comedy moves as fast in the spoken exchanges as in the sung ones. So much care, thought and effort has been put into this production: the quality shows in every note.

Originally, Fantasio was a tenor role; when the tenor he had originally booked for the opera’s première in 1872 accepted another role in America, letting him down, Offenbach re-scored the entire opera to transform Fantasio into a “trouser” mezzo. This was just the first of many hitches: the Franco-Prussian war delayed the première by two years; the initial run in Paris was only given ten performances before being dropped, much to Offenbach’s disappointment (and fury); manuscripts were mislaid... Fantasio risked slipping silently into the abyss of lost good ideas which threatens any work of art misinterpreted in its own time. The supreme irony is that Offenbach wasn’t trying to be controversial: indeed, he dedicated Fantasio to the notoriously conservative critic Eduard Hanslick, whose violent opposition to “the music of the future” – as pioneered by Wagner, Liszt and other progressives – was well known. Unfortunately, the unpalatable bit of Fantasio was Offenbach himself: his close association with Napoleon III and his German birth left him in a difficult social and political position in Paris during the Franco-Prussian war. Fantasio seems to have been, therefore, an unlucky and undeserving victim of anti-Offenbach sentiments in Paris in the early 1870s.

Thanks to Jean-Christophe Keck, however, this lovable rogue and incurable romantic – whose story blends Cyrano de Bergerac with comic misperceptions and fairytale romance, sparkling throughout with musical gems – is back with us for good.