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Filets of Sole Au Gratin
Heat a tablespoon melted butter in a sautoire, add one small, sound, white onion and cook for five minutes without browning, occasionally stirring; add six good-sized, well-peeled and thoroughly cleaned fresh mushrooms; stir well and let cook for five minutes; then add one and a half gills demi-glace; stir well and boil for five minutes. Thicken with one egg yolk diluted in a tablespoon cold milk; stir well while heating for one minute without boiling and keep warm on the corner of the range.
Lift up and skin the filets from a fresh three-pound flounder; cut each filet, crosswise, into three equal pieces, arrange in a sautoire, adding one teaspoon salt, half ounce butter, half gill white wine and a gill cold water. Cover the fish with a sheet of lightly buttered paper. Boil on the range for one minute and set in the oven to bake for fifteen minutes. Remove, lift up the paper, place the filets on a hot baking dish, pour the above sauce over the fish. Sprinkle a tablespoon grated Parmesan cheese over all, reset in a hot oven to bake for five minutes more. Remove, decorate the dish with thin slices of lemon and serve.
A short Escoffier-era fish course that depends on two things: a sauce enriched with egg yolk that must never boil, and fillets that are pulled from the oven before they dry out.
- The hinge: the yolk liaisonOnce the egg yolk goes into the demi-glace, keep the pan below a simmer. A minute of gentle heat thickens it; a boil scrambles the yolk and the sauce breaks into grains.
- GillsA gill is 4 fl oz, half a cup. So one and a half gills of demi-glace is 3/4 cup, half a gill of white wine is 1/4 cup, and a gill of water is 1/2 cup.
- SautoireA straight-sided shallow sauté pan. For the fish stage use one that can go from burner to oven and holds the pieces in a single layer.
- Demi-glaceReduced brown veal stock with espagnole; a strong homemade brown stock reduced by half is the practical substitute. Commercial glace de viande diluted to sauce consistency also works, but taste before salting, as it is concentrated.
- Oven temperaturesThe recipe gives no temperature for the covered fish, only 'the oven'; a moderate oven, roughly 350-375F, suits it. It does specify 'a hot oven' for the final pass, which today would be around 450F, or a broiler for two or three minutes so the Parmesan colors before the sauce splits.
- Cut the baking timeFifteen minutes will likely overcook flounder fillets cut into three pieces in a modern oven; eight to ten minutes at moderate heat should be enough. They are done when the flesh is just opaque and separates into flakes under light pressure.
- Sole, meaning flounderThe title says sole and the method says flounder, which reflects American fish-market naming of the period. Any thin flatfish fillet, or a firmer fillet such as fluke or plaice, behaves the same way.
- Buttered paperParchment cut to fit the pan and buttered on the underside. It traps steam and keeps the surface from drying; foil does the job but presses more heat into the fish, so check earlier.
- YieldA three-pound flounder gives roughly a pound of fillet. As written this is two generous servings, or three as a first course.
Fillets of a flat fish poached in wine, sauced, dusted with Parmesan and flashed under high heat is the standard au gratin formula of the professional French kitchen as it was codified in the late nineteenth century, and it turns up in American hotel cookbooks of the era. The tells here are all restaurant-brigade: the sautoire, the demi-glace kept ready as a mother sauce, the instruction to hold the finished sauce "on the corner of the range," which suggests a coal range with a cooler edge rather than a burner you can switch off. The slippage between the title's sole and the recipe's flounder was routine in American markets, where true Dover sole was an import and thin flatfish fillets were sold under the more flattering name; the practice is old enough that "lemon sole" and "gray sole" on a fish counter today are still flounders. The dish belongs to the moment when hotel French cooking was being written down for households that had a cook, a range and a supply of brown stock on hand.