Place a pan of water on the stove, about a third full, and bring to a simmer.
Whisk all the ingredients together in a heatproof bowl with a balloon whisk.
Place the heat proof bowl over the pan of simmering water and keep whisking until the curd has thickened and has lightened in colour (this takes around 7-10 minutes).
Remove from heat, tip the curd into a serving bowl and leave to cool.
Once cool, cover and place in fridge until ready to serve.
Store in the fridge, covered, for up to 3 days.
Notes
*This is a sweet-sour curd, it tastes sweet at first but you are left with a slightly sour taste afterwards. We as a family love this, but if you prefer something sweeter, then add another 3 tsp/15g sugar.This is a runny curd, ideal for drizzling over hot toast; but not the best for putting inbetween biscuits.For a smooth silky curd, pass it through a sieve once cooked, to remove the pieces of mandarin zest.Mandarins – Look for mandarins that are firm and heavy, and that are a bright orange colour.White sugar – I use granulated sugar and find the sugar granules dissolve perfectly fine, however you can use caster sugar (superfine sugar) if you prefer.Egg – a size large egg. I use the whole egg in this recipe, using just yolks will give a richer curd, but in this particular curd recipe I don’t think you notice it, and it saves wondering what to do with 1 egg white.