Beat the egg yolk with a few drops of vinegar. Add the oil a little at a time while constantly whipping (not too much at a time).
Continue until the mayonnaise has the desired texture.
Halve the chilli and remove most of the cores. Finely chop the chilli and stir it into the mayonnaise.
Season with salt and pepper and if necessary a little more vinegar.
Serve the tempura prawns on a bed of salad and pour mayonnaise on top.
