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Stir together soy sauce, water, lime juice, ginger, sugar, and scallion.
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Coarsely crush peppercorns and coriander with a mortar and pestle or heavy skillet. Sprinkle tuna with 3/4 tsp salt, then press spice mixture evenly all over fish.
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Heat oil in a 12-inch heavy skillet over medium-high heat until it shimmers, then sear tuna, turning once, about 6 minutes for medium-rare.
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Serve with dipping sauce.