While Lyric Opera gears up for its 35th anniversary with one of the most shamelessly conservative seasons ever offered by a major opera house, turn to Chamber Opera Chicago for the only 20th-century opera to be heard this fall, and a Chicago premiere at that. Gian Carlo Menotti is probably best known for his tuneful Christmas opera Amahl and the Night Visitors, but the old pun that "if you've heard Menotti, you've heard Amahl" is unfair to the Italian-born master's other operas, which are no less engaging, entertaining, and musical. The Hero is probably Menotti's least known work; it premiered 13 years ago in Philadelphia and has fallen into general neglect ever since. This is puzzling, for the libretto (Menotti's own) is clever, the action fast paced, and the music delightful but never heavy. The story concerns a man who falls asleep for over ten years and his wife's attempts to cash in on his world record for uninterrupted sleep. The comedy that ensues owes as much to Watergate as it does to Rip Van Winkle or Sleeping Beauty, as his wife tries to pass off tapes--complete with gaps--as proof of her claim. Tonight, Saturday, and next Friday and Saturday, September 15 and 16, 7:30 PM, and next Sunday, September 17, 3 PM, Ruth Page Auditorium, 1016 N. Dearborn; 822-0770.