OPEN the Garsington Festival programme-book, and the first thing you find is an advertisement for Hello! magazine - which tells you something about the sort of people you're probably sitting next to. But in the past 10 years - and yes, it is now a decade since Leonard Ingrams started sharing opera with his neighbours - Garsington has managed to transcend the social and the decorative. It's a deadly serious undertaking, with a track record that includes the rediscovery of Haydn's virtually forgotten stage-works and the re-evaluation of rare Strauss scores. And few things could be more serious (or potentially more deadly) than Lucio Silla, which opened there on Tuesday.

Mozart wrote this rambling score at 16; and for a teenager it is, of course, a prodigious achievement, with attractive arias and accomplished orchestration. But great theatre? No. The plot - a Roman emperor finds his conscience and forgives all - is creakily conventional opera seria, with a turgid libretto, a choked succession of da capo arias ( just one duet, one trio for variety), and a narrow vocal range. There's no bass role: just two not-very-weighty tenors, and everything else given to sopranos or soprano-castrato. The result is that the cast might as well be singing Suor Angelica for all the darker colouring they offer.

But at Garsington they make the most of it, with some fine singing from two ladies in trousers who deliver the female focus with near-Straussian voluptuousness: Anne Dawson as Cecilio and Elena Ferrari as Cinna. Thomas Randle in the title role doesn't make much of a vocal impression, but acts as though his life depended on it. Steuart Bedford is an astute and inspired Mozart conductor. And with a staging by Aidan Lang that plays the action cleanly against an austere slate-grey set (the costumes quasi- Paul Smith with a touch of toga), it's a stylish show, the boredom factor cut down by some welcome trimming to the text.

The one thing Garsington can't sort out is the weather. This is garden opera, semi-open, under canvas; and although the company's booking leaflet quaintly suggests that "shawls etc are advisable", what it really means is sou'westers for nights like Tuesday, when Force 9 gales tore at the tenting and the rain fell like Niagara. It's not just a matter of discomfort but of distraction. And since the chances of Garsington ever getting planning consent to build a proper theatre are zero, it will ever be thus. There's a Dunkirk spirit about wringing out your trousers in the interval, but I could live without it. So, I daresay, could the Garsington musicians.

Interval trouser-wringing is not a conspicuous feature of opera-going at Baden-Baden in Germany. There, they have a brand-new Festspielhaus, which opened for business last weekend, courtesy of our own Royal Opera - temporarily decamped to Germany to earn some money. It's a desperate measure, but will bring the company prestige: this residency is the highlight of a brand-new festival with ambitions to be a German Salzburg. In fact, it actually incorporates the Pfingstenfest, the Salzburg Whitsun, which, by means I don't pretend to understand, has sprouted legs and walked across the border.

The surprising thing is that Baden wasn't a German Salzburg already. In the 19th century it was the summer watering-place for the itinerant rich of Europe, with musicians (as always) following in their wake. Johann Strauss, Brahms, Clara Schumann, Pauline Viardot lived there. Berlioz wrote Beatrice and Benedict for the opening of the (old) town theatre. But nothing lasting grew from this concentration of great names, and Baden settled down to being the discreetly elegant Black Forest spa it is today; famous for its casino and colonic irrigations, but not otherwise a world centre of culture.

The new Festspielhaus may change that, although from the start, it's been a controversial project. It's a commercial venture, run without state subsidy, and its plan is to buy in projects from abroad, with no permanent company of its own and (its critics claim) no artistic policy to give the festival season coherence. It's also a big theatre for a small town, with 2,500 seats which, judging by last weekend, won't be easy to fill. But visually, the theatre is stunning: a square-cut, chic-ly modernist extension to the belle epoque facade of what was once the Baden railway station. And the sound is good, with an all-round resonance that amplifies the orchestra, but builds the voices too. It gave the whole Royal Opera forces last weekend a bloom and substance that I've never heard on home ground.

And I'm pleased to say that they were on dazzling form, with three shows on successive nights that so triumphantly showcased the British arts abroad (and so bravely denied the way the company has been brought to its knees in London), they brought tears of patriotism to my not-normally- jingoistic eyes. A pity Chris Smith wasn't there to see it all. It might have taught him something.

First came Aegyptische Helena, as heard at the Festival Hall, but heard much better here, with superb singing throughout the cast, and spectacular playing from the orchestra under Christian Thielemann. Then came the Richard Eyre Traviata, properly staged (as against last time round in London), with a magnificent Violetta from Elena Kelessidi, and a celebrity conductor in Placido Domingo - who wields a baton often enough these days to know what he's doing, and to do it competently, even if the result isn't always distinguished.

To complete the residency, though, there was a new production that won't been seen in Britain until this year's Edinburgh Festival, and won't hit London until next year. It was Verdi's I Masnadieri, staged by Elijah Moshinsky, conducted by Sir Edward Downes, and a challenge to the ingenuity of all concerned, in that it's one of Verdi's weakest works. It's adapted from Schiller's The Robbers, and its plot comes close to the King Lear that Verdi always meant to tackle, but with wayward sons instead of daughters, and a background of marauding brigands who sing eulogies to rape and pillage in politely cheerful 4/4 time. With an extreme dramatic foreshortening paring Schiller's narrative to the bone, the action is absurdly abrupt ("Where have you been?" "Burning Prague to the ground!" "Oh really?") and often motiveless ("Time this opera ended: I think I'll stab my girlfriend." Curtain.) Understandably, it's rarely done, and the full score has never yet been printed.

But as it was written for London and the original manuscript parts lodged in the Royal Opera archives, that didn't worry Edward Downes - until he found that Jeremy Isaacs had quietly flogged them in a moment of financial crisis. Luckily, he then discovered that they'd been copied on to microfilm. And from that film, the performance parts have been painstakingly reconstructed into music that works better than it should.

Downes has nurtured the whole project to life with the instincts of a master Verdian. And his conducting so forcefully proclaims a belief in the piece that it's hard to disagree. Elijah Moshinsky's production is clean and spare, but building into striking stage pictures which play the story for real (no irony), and side-step its implausibilities with tasteful artifice. There's handsomely assertive singing from Dmitri Hvorostovsky as the bad son Francesco, Franco Farina as the not-so-bad son Carlo, and Paula Delligatti as Amalia, the sympathetic hanger-on.

And from the strength of the orchestra and chorus, you'd never guess the appalling pressures to which these musicians are currently subjected. It's a valiant exercise in keeping up appearances. I hope they keep them up until Edinburgh.

Garsington Festival (01865 361636), to 5 July. Baden-Baden (0049 711 7804166), to 9 Sept.