Stir the fish sauce, sugar, and lime juice together, set aside. In another bowl, mix the ginger, lemongrass, and scallion whites, together set aside.
Heat a large skillet over medium heat. Add 1 1/2 tablespoons peanut oil to the skillet and season the fish with pepper. Stir-fry half of the lemongrass mixture and cook until fragrant, about 30 seconds. Lay 2 of the fish fillets, rounded side or skin side down and cook until browned and a spatula can easily be slipped under the fish, about 2 to 2 1/2 minutes. Flip the fish; add half the fish sauce mixture and half of the scallion greens. Cook the fish, swirling the pan, to coat the fish as the liquid thickens into a syrupy sauce, about 2 minutes.
Transfer fish to a platter and repeat with remaining ingredients. Wipe or wash out skillet before starting the second batch. Scatter the tomatoes and basil or cilantro over the fish and serve.
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