Wash and dry the oranges. Grate the zest of one orange then add 1 tablespoon to a medium-sized pot. Reserve the rest for another use.
Slice the oranges in half and squeeze out as much juice as you can. Pour ½ cup (125ml) juice in the pot with the orange zest. Store or drink any juice leftover.
Add the sugar, salt, and eggs to the pot. Whisk everything well to combine, then let the mixture sit for 10 minutes so the sugar can begin to dissolve. Whisk again.
Break the butter into small pieces and add them to the pot. Place the pot over medium heat on the stovetop and cook the curd to 175º to 180ºF (79º to 82ºC), stirring constantly. Once the curd has reached temperature, pour it into clean glass jars. Place the lids of the jars on top and slightly ajar until the curd cools to room temperature. Tightly secure the lids and store the curd in the refrigerator for up to one week.
Notes
The orange zest should be grated fine with a microplane-style zester.
You want to cook the curd over medium-low heat to avoid scrambled egg bits. If you do notice some bits of cooked egg, simply push the curd through a sieve before storing in jars.
Once you see steam rising in the pot, the curd is getting close to temperature.
Store orange curd in clean glass jars with tightly secured lids for up to one week in the fridge.