YOU ARE 25 years old. Among the 20 operas on your CV are The Barber of Seville, L’italiana in Algeri and La Cenerentola, three of the all-time great comedies. You are the king of comic opera, your greatest hits are the soundtrack of European life. What are you going to do next? Well, not much, if popular history is any guide. By the time most of us are thinking about getting a proper job, Gioacchino Rossini had invented the blueprint of Italian opera for the next couple of generations and, apparently, retired to bed with large quantities of champagne, truffles and tournedos, whence he issued a stream of bons mots and genial insults.

Legend relates further that after a few years he bestirred himself to invent the regrettable artform known as French Grand Opera with his William Tell, but even then neglect set in pretty soon. When a few years later he heard the Paris Opera was to stage Act II of Tell, he remarked: “What? All of it?” and hit the sack for good.

The last bit is more or less true — Rossini wrote Tell, his last opera, at 37 and lived for another 40 years — but what about that 12-year gap in the middle from 1817 to 1829? Most people have no idea that Rossini was ever anything but a writer of comedies, not surprisingly given the leap in quality between the Neapolitan opera buffa tradition he inherited and his own efforts. Partly this leap was due to the attention he paid to formal structures; partly it was his inimitable rhythmic vitality and mastery of the orchestra; and partly it was his genuine wit and a delight in playing with language and with the conventions of opera. The scene in The Barber where the characters’ means of escape (a ladder) is removed even as they rehearse an endless let’s-go routine is still one of opera’s most sublime comic moments.

Although his serious operas are almost totally neglected, their influence was immense: Donizetti, Bellini, Verdi and even Wagner could not have existed without them. Even before La Cenerentola, his last comedy, at 25, Rossini had made successful forays into serious opera with Tancredi and Otello. And in 1817 he landed a job in Naples, where he would write Armida, Mosè in Egitto, La donna del lago, Maometto

II and Zelmira — which did for the serious opera what he had already done for its comic cousin.

Well, judge for yourself. Zelmira, the last of his Neapolitan works, is being performed in a concert version at Edinburgh and it is likely to be an eye-opener: proof not only that Rossini’s apparently insouciant muse could turn its hand to pain and pathos, but a work that carries echoes of Beethoven in its scoring and hints of the darkness and passion of Verdi 30 years later.

I asked Elizabeth Futral, the gorgeous Louisiana gal who will sing the title role, if she had any idea why these operas are so rarely performed. “Not really!” she says, disarmingly. “I guess the comedies might be more accessible, but it’s been a real joy for a non-Rossini specialist like me to learn the role. It has a combination of coloratura challenges and graciousness that works really well once you wrap your voice round it, and it’s really gratifying to sing: Zelmira has some truly exquisite moments, and the orchestration seems really forward-thinking.”

Futral has made her name in the light coloratura repertoire, from Handel’s Semele via Mozart’s Susanna to Donizetti’s Lucia and others, and she says: “The way Rossini writes for the voice does seem to me significantly different from Donizetti, more harmonically based than melodically somehow, and I’ve had to adapt to the slightly lower tessitura.”

I wonder whether she ever gets sick of singing these bel canto crazies: “Oh well, a few of them are sensible and smart: Susanna, Adina in L’Elisir — they’re not all raging hysterics! — and Zelmira has plenty of dignity too.”

One thing that has stood in the way of these operas has been their often appalling librettos, and Zelmira is no exception: a tale of political machinations on the island of Lesbos involving the usual Trojans and Greeks. But what Zelmira did give Rossini was a noble and victimised heroine who let him demonstrate the feminist sympathies he shared with Handel and Mozart.

It’s not all Rossini owed to the northern European composers. He had spent his youth studying Haydn and Mozart, and it shows. Zelmira contains, alongside its vocal fireworks and orchestral vivacity, several recitatives that hark back to the Baroque, a romantic, Beethovenian introduction and an explosion of joy in the last act accompanied by oboes and piccolos that recalls Fidelio. But what stays in the mind is mostly its sheer brilliance, of invention, orchestration and spirit.

Zelmira was Rossini’s last Naples opera, and he knew that it would be played in Vienna, London and Paris, which accounts for some of its musical sophistication. It also makes superhuman vocal demands — Rossini wrote the title role to put his future wife Isabella Colbran through her paces, and the tenor roles, sung by Bruce Ford and Antonino Siragusa, are arguably even more demanding than Futral’s. At the time of composition (1822), nobody would have even thought about trying to produce the tenors’ ridiculously high top notes at full voice. The first to attempt it was Gilbert Duprez in William Tell: the composer said it sounded like “a capon whose throat is being cut”.

The rest is history. Earplugs might come in handy.

• Zelmira is at the Usher Hall, Edinburgh (0131-473 2000) on Saturday