Place the egg yolks in a large bowl along with the mustard, rice vinegar, sugar, fish sauce, a little salt and white pepper, and whisk well using a whisk or an electric hand mixer.
Now whisk the oil into the egg yolks a little at a time in a thin stream. Pour slowly enough to ensure the oil is constantly being whisked into the egg yolks. Pause occasionally whilst adding the oil and whisk the mayonnaise thoroughly.
Continue until all the oil has been added, then whisk in a little water. Whisk thoroughly again – this helps ensure the mayonnaise doesn’t separate and gives it a nice, smooth consistency.
Finally, season to taste with salt and, if desired, a little extra rice vinegar.
Refrigerate the mayonnaise immediately, where it will keep for 6–7 days.
Tip: The fish sauce and sugar should not be directly detectable – they simply give the mayonnaise a deeper umami flavour, reminiscent of the characteristic taste of Japanese Kewpie mayonnaise.