Split lobster in half lengthwise; remove stomach sacks and intestinal tubes. Remove and crack claws and joints. Separate tails from chests. Heat oil or butter in large skillet. Add lobster pieces, meat-side down; saute for several minutes, turning with tongs until shells are bright red.
Remove lobster and keep warm. Saute onion and garlic in skillet. Add remaining ingredients. Bring mixture to a boil; cook until liquid is reduced by half. Add lobster; simmer 8-10 minutes, basting every few minutes. Arrange lobster pieces on a warm platter and spoon sauce over them.