Put the nuoc cham and agar-agar in a saucepan and whisk until combined. Bring to the boil over a high heat. Simmer for 3 minutes then pour the liquid into a shallow tray. Carefully place the tray in the fridge for the agar-agar to set.
Once cool and firm, un-mould the jelly and transfer to a blender (you can also use a hand-held blender with a bowl). Blend the jelly until a smooth gel is formed. Put in a bowl and whisk in the ót tuóng. Transfer to a squeezie bottle or a lidded container and store in the fridge where it will keep for up to 2 weeks.
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