How to Make It
In a mortar, lightly pound the lemongrass. Arrange the fish on a work surface and pat dry. Stuff the cavities with the lemongrass. Brush 3 tablespoons of the cornstarch slurry over one side of the fish. Spread 1/4 cup of the salt over the slurry to form a thin crust. Let stand at room temperature until the crust is dry, about 20 minutes. Carefully turn the fish over and repeat with the remaining 3 tablespoons of slurry and 1/4 cup of kosher salt. Let stand for another 20 minutes, until completely dry.
Light a grill. Carefully place the tilapia on the grill and cook over moderately low heat, turning once, until the fish is just cooked through, about 20 minutes. Transfer the tilapia to a work surface. Using your fingers or 2 forks, peel off the tilapia skin. Transfer the fish to plates and serve with the Spicy Citrus Dipping Sauce.