How to Make It
In a medium saucepan, melt the butter. Add the onion and cook over moderately low heat until softened, about 4 minutes. In a small bowl, whisk 1 tablespoon of the cornstarch into the coconut milk until smooth. Add the coconut milk to the onion and cook over moderately high heat, whisking, until bubbling and thick, about 2 minutes. Remove from the heat and let cool to room temperature. Stir in the crabmeat and season with salt. Roll rounded tablespoons of the mixture into 20 small balls. Transfer the balls to a parchment-lined baking sheet, cover and freeze until firm, about 1 hour.
In a large bowl, toss the panko with the sesame seeds. In another large bowl, whisk the egg whites with the remaining 1 tablespoon of cornstarch until smooth. Dip the frozen crab balls into the egg mixture, letting any excess drip off. Dredge the crab balls in the panko mixture to coat thoroughly. Return the balls to the baking sheet and let stand at room temperature until thawed but not too soft.
In a small saucepan, combine the sugar, lemon juice and ginger and bring to a boil over moderately high heat, stirring, until the sugar dissolves. Strain into a small bowl.
Peel the grapefruits with a sharp knife, removing all of the bitter white pith. Cut in between the membranes to release the grapefruit sections. Transfer to a fine strainer set over a bowl and let stand for 15 minutes to drain as much as possible.
Transfer the grapefruit sections to a medium bowl and gently toss with 3 tablespoons of the ginger syrup and the chile. Let stand for 5 minutes. In another small bowl, mix the remaining 1 tablespoon of ginger syrup with the soy sauce.
In a medium saucepan, heat 1 1/2 inches of oil to 300°. Set a rack over a baking sheet. Fry the crab balls, about 4 at a time, until golden brown, about 1 minute; transfer to the rack to drain while you fry the rest.
Mound the grapefruit salad in shallow bowls or plates. Arrange the crab balls next to the grapefruit. Pour a puddle of soy syrup on the other side of the crab balls and serve at once.