Place fish without head in a large, shallow frying pan. Pour off milk, add pepper, lemon slices, bay leaf, onion quarters, carrot and parsley. Heat carefully, gently bubble for 5 min, cool. Lift out fish, pour liquid through a sieve. Skin and bone fish, then divide into small pieces.
Heat butter (1), add flour, toast 2 min until flour is lightly golden. gradually add fish liquid, stirring meanwhile until mixture is smooth. Continue stirring for 2 min until mixture comes to a boil, make 1 min, remove from stove, pour in fish.
Heat butter (2), saute onion slices in it for 3 min. Spread onion slices evenly on the bottom of a shallow dish. Put fish mixture and eggs on top. Pour half of the sauce over it. Spread potato slices evenly on top. Pour remaining sauce on top. Sprinkle with bread crumbs mixed with cheese. Bake in a heated oven at 180 degrees until the topping is golden brown.
Serve with: leaf salad