Eliogabalo, La Monnaie, Brussels

Heliogabalus ranks as one of Rome's imperial rotten eggs: a mere stripling hoist to the purple by vox pop and the Pretorian Guard, loathed by the posh for his outrageous gay pranks and elevating ex-slaves and women to the senate - such brazenness would have endeared him to Mrs Pankhurst - he got dumped for his cousin while still in his teens.

Heliogabalus ranks as one of Rome's imperial rotten eggs: a mere stripling hoist to the purple by vox pop and the Pretorian Guard, loathed by the posh for his outrageous gay pranks and elevating ex-slaves and women to the senate - such brazenness would have endeared him to Mrs Pankhurst - he got dumped for his cousin while still in his teens.

Eliogabalo (1668) is the last surviving of Francesco Cavalli's 30-plus operas. Its sophistication shows at every turn: we are beyond Monteverdi and in the run-up to Handel. Censored in Venice, mainly for its regicide - shades of A Masked Ball - it never got performed. It's a tasteful coup for La Monnaie to be giving effectively its European premiere. René Jacobs prises astonishing sounds from dulcian, harp, guitar and scintillating strings, plus flurries of tambourine, castanets and a dinky military tenor drum, all flourished with panache by Concerto Vocale's percussionist, Michael Metzler.

Act II's gold-leafed trelliswork looked like a gilded goose, but due to Vincent Boussard's erratic direction this was no golden egg: more platinum. The staging got better, but it was a tribute to four fine male singers and a handful of young female and travesti performers that they survived the footling, maladroit antics of the outset. Eliogabalo (a castrato role) was sung with zest by the Spanish mezzo Silvia Tro Santafe: she tends to belt, but her aria while Giuliano (Lawrence Zazzo) attempts to bump her/him off, was a gem of pathetic irony.

Annette Dasch (Flavia Gemmira) was the most subtle and mature of the female quintet. A young Italian, Giorgia Milanesi, showed promise as the fledgling Alexander Severus. Greek-born Mario Zeffiri (nurse in drag) and American Jeffrey Thompson (sexy not-quite boyfriend) blossomed when freed of silly antics. The best comic was Sergio Foresti - a superb bass of seasoned Baroque/ Serva Padrona slickness. A pity the pretty-boy and religious rows rarely gain hold, and the murder occurs offstage; without its fresh, cynical twist, this feels like Radamisto shorn of political tension. Sex bred Heliogabalus's tyranny; but not horny wife-chasing.

It was pure joy to decamp to Antwerp's Vlaamse Opera, for a bit of real directing. The David McVicar/Michael Vale production of Idomeneo remains a masterpiece, and this Flemish company thrilled - not least Kurt Bikkembergs's massed choruses, fabulously moved, shiveringly lit and movingly sung. Deon van der Walt performed nobly in the Bruce Ford/Philip Langridge title role; Britons Russell Smythe and Sarah Fox brought pathos to Arbace and Ilia; Lyne Fortin was a galvanising Elettra. Kristine Jepson as Idamante - rarely as fluent as Magdalena Kozena but as fine - sealed this superb staging, energetically paced by Gerard Korsten.