In large saucepan or Dutch oven over medium-high heat, fry onions and salt in butter until golden, about 10 minutes. Stir in flour, mace, celery seed and pepper; fry, stirring, for 1 minute. Stir in milk and, stirring, bring to boil; reduce heat and simmer for 3 minutes. Remove from heat; stir in parsley.

Holding knife at 45-degree angle, thinly slice fish into broad slices; set aside.

With mandoline or knife, very thinly slice turnips; set aside.

Thinly slice one-third of the potatoes. In buttered 12-cup/3 L baking dish, arrange layer, alternating and overlapping sliced potatoes and half of the turnips; top with half of the onion sauce, then half of the fish slices. Repeat once with another third of the potatoes and remaining turnips, onion sauce and fish. Slice remaining potatoes and arrange as top layer.

Cover with buttered foil, place on baking sheet and bake in centre of 375F/190C oven for 1 hour; uncover and continue baking until potatoes in centre are tender all the way through and top is golden, about 45 minutes.

Makes 10 to 12 servings.