Cook lobster tails in boiling water for 10 minutes or until done.
Remove meat (keeping shells intact); cut in 1-inch pieces.
In saucepan, cook onion with chervil in butter until tender.
Add soup, sour cream, and lobster.
Heat; stir now and then.
Spoon lobster mixture into shells.
Top with cheese.
Broil 4 to 5 inches from heat until cheese melts.