Il rè pastore was the last operatic work that Mozart completed before Idomeneo, and though there is a clear shift in style and level of achievement in the six years that separate them, it is very interesting to compare the 19-year-old composer to the later operatic master that we are more familiar with. Written as a commission for a visit to Salzburg by the Archduke Maximilian Franz, Mozart chose a libretto by the then rather old-fashioned Metastasio on a pastoral theme very popular at the time. The historically-based story is very simple: a young shepherd called Aminta is, unbeknown to himself, in fact the rightful heir to the throne, and Alexander the Great sets about finding him and restoring power to him. This however means that he can't marry his lover Elisa, a shepherdess, and is required to marry princess Tamiri instead, who is herself in love with Alexander's aristocratic assistant Agenore. In the end, the correct couples are united, and Alexander gives everyone their own kingdom.

Though in some ways it could be considered an opera seria, at the time of composition, this piece was called a "serenata", intended as a festival piece, with minimal staging (it was given here as a semi-staged performance) and a generally joyous atmosphere: that it lacks the more complicated and more interesting aspects of Mozart's later operatic style is not, then, just down to a lack of artistic maturity. What he does have already is the ultra-ripe busyness and sensuous beauty in the orchestral accompaniment, and also the constant experimentation with convention and style, with the accompanied recitatives emerging as some of the most accomplished and innovative portions. What he lacks at this age is sufficient contrast in mood and pace, and characterisation is far less individuated and more dependent on context than on a deep and nuanced view of each role as a human being to be portrayed on the stage. Additionally, he has not yet discovered the importance of ensembles, so we only get a brief duet at the end of Act I and a final (wholly glorious) chorus for all five principals at the end of Act II.

One number at least could be from the mature Mozart's pen: the Act II rondò aria "L'amerò, sarò constante", which Mozart imbues with the layered meanings, wistful poignancy, yearning vocal arches and dramatic ambiguity in the setting of text that characterises his mature operatic writing. Additionally, the orchestral accompaniment is simply ravishing, with its melting solo violin line, luxuriant woodwind writing, and warm flutters and ripples from the strings.

This opera requires three exceptionally agile sopranos, and two tenors with coloratura facility, so finding a cast with sufficiently contrasted voices is always going to be a challenge. Rolando Villazón is clearly still finding his footing after returning from a rest following a vocal crisis. The lower half of the voice is as firm and exciting as it ever was, fruity and vibrant and with an emotional surge that gives him a magnetic presence. But above the passagio he was very careful, and though the notes were still there, either by design or by necessity, the tone would slim dramatically and sounded pressed and slightly nasal. It's obvious, though, that this is still an important voice. The role of Alessandro is a rather low-lying tenor part, and he seemed comfortable in it – surprisingly so in the demanding coloratura, which didn't exactly sound tossed off, but was far above the level that is often heard in this repertoire. He proved also that he could modify his approach to be sensitive to the correct style whilst retaining his individuality and artistic personality. A tenor without confident high notes is never going to have an easy time of it, but there are roles for him still, particularly in early music, and he still has a lot to offer audiences.

Martina Jankova shone as Aminta, with her uncomplicated, very silvery, bell-like sound, effortlessly in tune, with a tight, fast vibrato that gives it an affectingly vulnerable edge. The earnestness of her intention and complete belief in what she is singing come across very strongly, and this is reflected in her compellingly direct acting, effective and moving even in this reduced theatrical setting. The aforementioned Act II rondò inspired a rapt concentration in the audience and was very beautifully sung indeed, with a daringly pared-down tone so that the voice was sometimes on the edge of a flutter. Here and throughout, a large number of ornaments were added to the vocal line, all very much in style, and cadenzas were freely recomposed to suit each singer's strengths and character.

Although the voice is very well controlled in the coloratura, Eva Mei didn't quite have the requisite freshness for the child shepherdess Elisa, and in an effort to sound bright and sweet the voice sometimes became breathy and a little thin; for most of the time it felt like she was singing at one dynamic level lower than was comfortable. She sounded much more focused and brilliant in the few moments when she allowed the voice to expand to its full power, but overall this didn't seem like quite the right role for her. Sandra Trattnigg in the more minor role of Tamiri struggled to cover the notes in her difficult Act I aria, and though she improved in Act II, there was a considerable graininess in the voice that made me wonder if she was unwell. Benjamin Bernheim has a very clear, bright tone and a very good technique, though I felt his performance as Agenore was too ironically inflected to allow him to create a believable or sympathetic character.

William Christie conducted the Orchestra La Scintilla der Oper Zurich in a carefully considered and dedicated performance, that bustled along without ever rising far above the functional, with timbral and dynamic contrast lacking and risk-taking left to a minimum. All of the voices could easily have dealt with a bigger orchestral sound, and indeed usually would have benefited from it – there was often too much of a sense of dutiful accompaniment rather than real ensemble work between the singers and instrumentalists. Still, Christie could never be criticized for being unmusical and this was an entirely respectable reading of a very interesting milestone in Mozart's oeuvre.