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Filet of Mackerel, Au Gratin
Cut head and fins from a three-pound mackerel, split in two through back, remove spinal bone, then cut each half in three slanting equal pieces. Finely chop a white onion, three medium fresh mushrooms and three branches parsley, sprinkle half of it in lightly buttered baking dish, pour in a half gill white wine and lay fish over one beside another, cut side up. Season with a half teaspoon salt and three saltspoons paprika, spread balance of hash over fish, then pour over one light gill demi-glace. Dredge over three tablespoons bread crumbs, arrange a few little "dots" of butter on top, then set in oven for twenty-five minutes, remove, place dish on another and serve.
A straightforward baked mackerel that depends on two things the original assumes you have on hand: demi-glace and an oven you already know the temperature of. Sort those out and the dish is twenty minutes' work.
- The hinge: don't bake it for 25 minutesSix slanting pieces off a 3 lb mackerel are thin, oily fillets, and the recipe's timing dates from ovens whose heat was guesswork. Bake at 400F and check at 12 minutes; the flesh should just flake and still look moist at the thickest point. Mackerel dries and turns dense fast once it goes past that.
- Oven temperatureNone is given. An au gratin finish wants brisk heat to brown the crumbs before the fish overcooks, so 400-425F. If the top is pale when the fish is done, run it under the broiler for a minute rather than leaving it in longer.
- Gills and saltspoonsA gill is 4 fl oz, so a half gill of white wine is 1/4 cup and "one light gill" of demi-glace is a scant 1/2 cup. A saltspoon is 1/4 teaspoon, making three saltspoons of paprika about 3/4 teaspoon.
- Demi-glaceClassically espagnole sauce combined with brown stock and reduced by half; a sauce more likely to be standing ready in a professional kitchen than a domestic one. Good jarred or frozen demi-glace works, or reduce 1 1/2 cups strong brown stock to a scant half cup. Thinning a bouillon cube will not do it; the sauce is meant to be glossy and cling to the fish.
- The "hash"This just means the chopped onion, mushroom and parsley mixture, used raw. Chop it fine so the onion softens in the same time the fish takes; large pieces will still be crunchy at 12 minutes.
- Boning the fishSplitting through the back and lifting the spine out is easier on a whole mackerel than filleting from the belly, but a fishmonger will do it. Ask for two fillets from a 3 lb fish, skin on, and cut each into three pieces on the diagonal. Run a finger along the fillet for stray pin bones.
- Buttered dish, cut side upUse a shallow gratin dish just large enough to hold the six pieces in one layer without crowding -- the wine and demi-glace should come partway up, not cover them. "Cut side up" means skin down against the dish.
- Yield and serviceServes three as a main course, four as a first course. "Place dish on another" appears to mean setting the hot gratin dish on a larger underliner plate before it goes to the table; a folded napkin under it does the same job.
The formula is textbook au gratin in the older sense: not a cheese-topped bake, but a dish finished with breadcrumbs and butter and browned in its own sauce, the crumbs there to form a crust rather than to bulk it out. The vocabulary -- demi-glace, saltspoons, gills -- belongs to the hotel-and-restaurant French that ran through American cookery of the period, used without explanation. Mackerel is a telling choice: cheap, abundant on both Atlantic coasts, strongly flavoured, and often paired with something sharp or savoury to cut its richness, which is the work the white wine and the mushroom-onion-parsley mixture do here. The sequence of instructions reads like a dictated professional method: trim, portion, layer, season, sauce, crumb, bake, send.