Too many opera singers who make the big leap into crossover popularity can't wait to sell out. Not Cecilia Bartoli, Italy's most precious gift to opera since Luciano Pavarotti. Having noticed that her fans will happily listen to her sing most anything, Bartoli decided some time ago to serve both art and commerce by following her own impeccable musical instincts. The mezzo-soprano's latest recital disc, Dreams & Fables (Decca), is a wonderful discovery: eight forgotten arias by Christoph Willibald Gluck, written when this 18th-century composer was busy supplying the courts of Europe with florid Italian operas. These works are unknown today, partly because fashions change, but more likely because such fiendishly difficult music can come to life only when sung by a virtuoso like Bartoli. She is in spectacular voice, and is passionately involved with these sorely beset heroines.