Heat oven to
325F (160C). Butter a baking dish about 12 x 7 x 2 inches (3.5 x 18 x
5cm). Cut fish into serving size pieces. Lay in large skillet. Add
celery leaves, parsley, thyme, bay leaf, lemon juice, peppercorns and 1
tsp salt. Add boiling water to just cover. Bring to a boil, turn down
heat, cover and simmer about 5 minutes or just until fish flakes easily
with a fork. Lift fish out of water. Strain cooking water and measure
out 1 cup (250ml) to use in sauce. Melt 3 tbsp butter in same skillet
and add onion. Cook gently 3 minutes, stirring. Add mushrooms and cook
and stir 2 minutes more. Remove from heat and stir in pimento. Melt 1/4
cup butter in saucepan. Sprinkle in flour and stir to blend. Remove from
heat and add the 1 cup fish cooking water all at once. Add 1/2 tsp salt,
1/4 tsp pepper, tarragon, marjoram and chervil. Stir to blend and return
to moderate heat. Cook until boiling, thickened and smooth, stirring
constantly. Beat cream and egg yolks together with a fork and stir into
hot mixture. Heat but do not boil.
Put a little
of the sauce in the prepared baking dish. Lay fish pieces in sauce and
sprinkle onion-mushroom mixture over. Top with remaining sauce. Add
bread cubes to 2 tbsp melted butter and toss to coat bread with butter.
Sprinkle over casserole. Bake 30 minutes or until bubbling and lightly
browned on top.