Heat the oven to 400°F with a rack in middle position. In a small bowl, stir together ⅔ cup water, the chili-garlic sauce, sugar, fish sauce and soy sauce until the sugar dissolves. In another small bowl, stir together the cornstarch and 1 tablespoon water.

In a medium saucepan over medium-high, heat 2 tablespoons of the oil until shimmering. Stir in the ginger and lemon grass and cook until fragrant, about 10 seconds.

Pour in the chili-garlic sauce mixture and bring to a boil. Stir the cornstarch mixture to recombine, then stir into the sauce. Reduce to medium-low and simmer, stirring constantly, until thickened, about 3 minutes. Remove from the heat, stir in lemon juice and ½ cup basil. Cover to keep warm.

Season the fillets on both sides with salt and pepper. In a 12-inch oven-safe skillet over medium-high, heat the remaining 1 tablespoon oil until just beginning to smoke.

Add the fillets flesh side down, reduce to medium and cook until golden brown, about 4 minutes. Transfer the skillet to the oven and cook until the thickest part of the fillets register 115°F to 120°F or are nearly opaque when cut into, 4 to 6 minutes. Remove from the oven.

Using tongs, carefully peel off and discard the skin from each fillet. With a wide metal spatula, transfer the fillets to a platter, turning them browned side up. Pour half of the sauce over the fish and sprinkle with the remaining 1 tablespoon basil. Serve the remaining sauce on the side.