To start the hollandaise, gently heat the butter and lemongrass in a small saucepan until the butter is melted. Remove from the heat, cover, and set aside for 2 to 3 hours. (Keep it in a warm place so it doesn't solidify.)

To make the fish fingers, melt the 2 tablespoons butter in a pan, add the scallions, garlic, ginger,and chili, cook gently for 5 minutes until softened, then transfer to a bowl and stir in the crab, cilantro, and fish sauce. Add the potato and beaten egg yolk and mix well to combine. Season to taste with salt and freshly ground black pepper.

Divide the mixture into 12 pieces and form each into a 1 x 4 inch rectangle. Beat the egg white lightly with a fork, mix the breadcrumbs and coconut together, and place in separate shallow bowls. Coat each fish finger in the egg white and then coat evenly in the crumb mixture. Set aside on a tray until ready to cook.

Heat a little butter or oil in a skillet and cook the fish fingers in batches, for 3 to 4 minutes or until golden brown all over, then transfer to a baking tray and keep them warm while you complete the hollandaise.

Put 2 tablespoons of the lime juice, 2 tablespoons water, lime zest, peppercorns, and shallot in a small pan and boil until reduced to 1 tablespoon. While still warm, strain through a fine sieve into a bowl set over a pan of hot but not boiling water.

Add the egg yolks and beat well with a whisk. Continue beating and add the butter in a thin steady stream (leaving the sediment and lemongrass pieces in the bottom of the pan) until all the butter has been added and the hollandaise is thick and glossy. If it is very thick, beat in a spoonful of hot water.

Season to taste with salt, pepper, and more lime juice if needed and remove from the heat.

In a separate bowl, beat the egg whites until stiff but not dry. Stir one-third of the egg white into the hollandaise, then carefully fold in the rest.

Serve immediately with the fish fingers.