Updated Monday, October 25th, 2010
Prepare marinade: combine first 7 ingredients in a heavy, non-corrodable saucepan, and simmer gently for 30 minutes. Strain, pressing on the solids to get all possible liquid. Measure and if necessary add water to make 3/4 cup. Combine with olive oil and set aside.
Layout shark in a single layer on freezer paper or other heavy paper. Combine flour, mustard seed, salt, and pepper and distribute half of mixture evenly over fish. Pound it in. Turn fish and repeat.
Heat a half-inch layer of oil in a wide, heavy skillet and fry the fish over fairly high heat until the fish is completely cooked and the seeds are toasted and brown. Turn once with tongs (do not pierce the fish). As each slice browns, transfer to absorbent paper to drain.
Put the fish in a deep serving platter and pour the marinade over it. Cover with plastic wrap and let marinate at room temperature for two hours or so. Baste frequently. At serving time cut tomatoes into thin slices and layer them with the fish. Garnish with the remaining basil and serve at once. This can be prepared a day in advance and held in the refrigerator. Just don't add the tomatoes until serving time, and be sure the dish has returned to room temperature for maximum flavor.
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