Drain fish and mash in a large bowl. Mix in nuts, cream, matzo meal, butter, lemon juice, onion powder, salt and pepper. Beat egg yolks and fold in. Beat egg whites stiff and fold in. Place in well greased loaf pan or 9 inch tube pan and spread evenly.
Cover pan with aluminum foil. Bake at 350 degrees until set and firm to the touch, about 1 1/2 hours. Loosen sides with knife, allow to set for a few minutes and turn out onto serving platter. Serve with tomato and mushroom sauce. Serves 6 to 8.