Nobody does special projects quite like Cecilia Bartoli – each one with at least a few premiere recordings, and each seemingly more elaborate than the last. Mission is no exception, having been preceded by a whimsical YouTube video series and even inspiring a new book by American detective novelist Donna Leon. The centre of all this activity? Agostino Steffani: composer, priest, diplomat and possible spy, whose name has fallen into obscurity but who, according to Bartoli and company, might just be the “missing link” between Monteverdi and Vivaldi in the development of Italian opera.

It’s hard to argue with their evidence. This double disc not only showcases Bartoli at her intense and virtuosic best; it’s an immersive musical experience, whose interest lies not merely in the novelty and rarity of the repertoire, but in its genuine brilliance. Gorgeous melodies, tireless musical invention, and a deft sense of theatre leap out at every turn (it’s hardly surprising to discover how heavily Handel was influenced by Steffani, even incorporating some of the latter’s compositions into his own works) and while the program is long, there’s little chance of fatigue.

Bartoli’s expressive palette is as colourful as Steffani’s own, and this music – much of it originally written for soprano castrati – is ideally suited to her talents. From the exquisitely tender Amami e vederai to the bellicose coloratura of Svenati, struggiti; from the thundering tempest of La cerasta più terribile to her ecstatic duetting with sweet-toned countertenor Philippe Jaroussky, Bartoli is in galvanising form. The vocal idiosyncrasies which have tended to divide listeners – breathy phrasing, hyper-staccato coloratura – haven’t disappeared, but nor do they overwhelm, and Bartoli’s glossy tone and inimitably characterful delivery come gloriously to the fore.

The Coro della Radiotelevisione Svizzera also make a strong impression, and the playing of period orchestra I Barocchisti under Diego Fasolis is superlative, vibrantly highlighting the variety of orchestral writing and further bolstering the case for Steffani’s inclusion among the Big Names of the Baroque. Bartoli’s continuing efforts in musical detection have had their ups and downs, but this is one of her most fruitful investigations yet. One final word of advice: there’s a jewel case release coming, but spring for the limited hardcover edition, with its extensive historical and biographical notes. Steffani’s life story really is the stuff of novels, and just as fascinating as his music.