Whisk the eggs with the water and fish sauce, then strain the mixture. Straining is the trick to that silky, lump-free texture.
Pour into a heatproof bowl or stone pot. Skim off any bubbles on top so the surface steams smooth.
Set the bowl in a steamer over simmering water and cover. Steam gently on medium-low for 10 to 12 minutes.
Check that the center is just set, soft and jiggly, not firm. Low heat is what keeps it custard-y instead of rubbery.
Drizzle with sesame oil and finish with sesame seeds and scallions. Serve warm straight from the bowl.