This sauce goes deliciously with all types of grilled meats.
Roasted sticky rice powder – often called roasted (or toasted) glutinous rice powder – is sold in specialist Thai shops. It's also easy enough to make your own, and I've given the method in the recipe. You won't need the full amount, but the leftovers can be stored in an airtight container for a couple of weeks, or frozen for longer.
Make the roasted rice powder. Put the raw sticky (glutinous) rice in an unoiled skillet and place it over a medium flame. Shake the pan constantly until the grains are lightly toasted and fragrant.
Cool the rice, then grind it to a rough powder in a food processor or blender; it should have a slightly grainy texture – do not grind into flour. Store in an airtight container.
Put the sugar into a bowl and add the fish sauce and lime juice. Stir to dissolve the sugar, then mix in the rice powder and chilli flakes.
Halve the shallots. Thinly slice the shallot halves and spring onion, and roughly chop the fresh coriander. Add the shallots, spring onion and coriander to the fish sauce and sugar mixture and stir well. Taste for seasonings and adjust, if necessary.