1. Prick eggplant all over. 2. Char over a gas flame. 3. Place in a plastic bag. Let stand 10 minutes. 4. Peel. Mash pulp. 5. Heat butter in a saucepan. 6. Add flour. Cook 1 minute. 7. Whisk in milk. Simmer until very thick. 8. Add eggplant. Cook 5 minutes. 9. Remove from heat. Stir in cheeses. Season. 10. Beat until very thick.