Set the sugar in a pot, and zest the oranges over the sugar. Use your fingers to rub the zest into the sugar until you feel it begin to release its oils and become like wet sand. Add the orange and lemon juice.
In a separate bowl, whisk together the eggs with a pinch of salt.
Slice the butter and place it into a bowl and set a fine mesh sieve over it.
Combine the eggs into the pot, add the starch and whisk well to combine. Set over medium low heat and begin cooking.
Use a rubber spatula or wooden spoon to stir the curd as it cooks. You’ll keep stirring as it warms and thickens. It will thicken on the bottom first, keep stirring until it reaches about 165 F (it should cover the back of a spoon).
Pour the thickened curd into the bowl through the sieve. Scrape it to get as much curd out as possible, and scrape the bottom of the sieve too.
Stir until the butter fully melts into the curd and you have a runny, silky curd. Transfer to a heatproof jar and let come to room temperature. Then set it in the fridge to store.
Curd will keep for about 2 weeks in the fridge.