Mix together the 2 tsp flour and 2 tsp butter; reserve. Butter generously a baking dish large enough to hold fish. Arrange fillets, covering with onion and lemon slices, season with salt and pepper, thyme and add bay leaf piece. Pour in 1-1/2 C Champagne. Place fish in preheated 400 degree oven, checking after 10 minutes. Fish cooks very quickly this way and cannot be allowed to brown. Remove when milky white and flakes to fork. Discard lemon and onion but drain juice into small sauce pan. Keep fish warm. Concentrate juice over stove, thicken with dabs of flour-butter mix. Add 1/2 C champagne and egg yolks. Thicken beating over double boiler or over low fire. Cool sauce quickly when done. Spread over fish and serve. Recipe date: 12/11/87