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1
Heat the oven to 220°C. Cut four large heart shapes from parchment paper, each approximately 45 cm at its widest point. Brush both inner faces lightly with neutral oil or clarified butter. Set aside on two heavy baking sheets.
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2
Melt the butter in a heavy saucepan over moderate heat. Add the shallots and mushrooms and sweat without colour until the shallots are translucent and any mushroom liquid has evaporated, approximately 5 minutes. Add the flour and stir to form a roux; cook for 1 minute. Add the white wine, raise the heat briefly, and reduce by half. Add the warm fish stock gradually, whisking to prevent lumps, then add the thyme. Simmer, stirring, until the velouté coats the back of a spoon thickly — it must not run when a finger is drawn across the spoon.
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3
Remove the thyme sprigs. Stir in the cream and return to a simmer for 2 minutes. Fold in the crabmeat and shrimp and season with salt and white pepper. The finished sauce must be thick enough to mound slightly and cling — a sauce that pools freely will saturate the parchment. This is where the dish lives or dies: a sauce with insufficient body will render the parchment sodden and the fish steamed in liquid rather than glazed by it.
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4
Season each pompano fillet lightly with salt and pepper. Place one fillet on the right half of each heart, slightly below the centre. Spoon a generous portion of the crab and shrimp velouté over each fillet, distributing the seafood evenly.
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5
Fold the left half of each heart over the fish. Beginning at the top of the heart, fold and crimp the curved edge in tight, overlapping pleats, each pleat folding back on the last. Continue around the full curve to the pointed base, then twist the point firmly to lock the seal. Press the entire seam. There must be no gaps — hold each package up to a light source; no gap should be visible at the seam. An incomplete seal will allow steam to escape and the package will not puff.
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6
Place the sealed packages on the oiled baking sheets and transfer immediately to the 220°C oven. Bake for 12 to 15 minutes. The packages are ready when the parchment is puffed dramatically — domed and taut — and golden-brown at the edges. Do not exceed 15 minutes; the sealed steam environment cooks the fish efficiently and overbaking dries it.
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7
Transfer each package at once to a warm dinner plate, still sealed and fully puffed. Bring to the table immediately with a lemon wedge and scissors. The package is opened at the table: the aromatic steam rises on cutting — this is the dish's signature moment and it cannot be replicated if the fish is plated in the kitchen.