How to Make It
In a large bowl, whisk the eggs with the stock, fish sauce and salt. Pour the mixture into four 10-ounce heatproof bowls or ramekins. Arrange the bowls in 2 large, wide pots with lids. Add enough boiling water to the pots to reach two-thirds of the way up the sides of the bowls. Bring the water to a simmer, cover and simmer gently until the custards are slightly wobbly in the center, 6 to 7 minutes.
Divide the shrimp and scallion among the bowls, cover and steam until the shrimp are cooked through and the custards are set, 3 minutes longer. Garnish the custards with sesame seeds and sesame oil and serve warm.