Cut fish across to make serving portions. Dip portions into flour, then into an egg wash made of 2 eggs, finally into the bread crumbs.
Fry in 7tablespoons of the butter till golden brown. Rather undercook them than overcook. Reserve the butter.
Wash the spinach 3 times, cook in salted water, and drain. Add pepper and purée the spinach; there should be 1½cups. Take 2tablespoons of the butter in which the fish was cooked and mix it with puréed spinach.
Cook potatoes in salted water; peel them and rice them while warm. Add the other 2 eggs, 2tablespoons butter and the mace; mix well.
Grease an ovenproof platter with remaining 1tablespoon of butter. Make a potato border around the edge of the platter with potatoes piped from a pastry tube. Spread spinach purée in middle of platter. Place fried fish on top of spinach bed. Cover everything except the potato border with the sauce. Brown under the broiler for a few minutes.