Zest and juice the oranges and half the lemon. You need to end up with 200g fresh juice.
Pour the citrus juice, zest and sugar into a medium saucepan. Turn the heat on and whisk everything together whilst bringing to a gentle simmer then remove from the heat.
Break the eggs and egg yolks into a medium sized mixing bowl, weigh to make sure they make up to 240g. Lightly beat together then pour a splash of the hot orange mixture into the beaten eggs, whisking vigorously to avoid the egg scrambling.
Gently pour the rest of the mixture into the eggs in a steady stream whisking all the time until the eggs and citrus mixture is completely combined.
Pour it all back into the saucepan, along with the salt, and turn the heat to lowest setting. Stir the curd with a wooden (or silicone) spoon, making sure to swirl gently into the corners, touching the bottom of the saucepan so the curd doesn’t settle there otherwise it will start to curdle.
Continue stirring until the curd starts to thicken which can take at least 15 minutes. You don’t want it to boil which will be when the mixture gets past 80°C (176°F) so if you have a digital thermometer you can keep an eye on it as well as feel for the turn in texture. The curd is ready when it starts to feel a little heavier and thicker to stir, almost like a pourable custard. It should coat the back of your wooden spoon. Dip it into the curd and run a finger down the back of it, if a clear line is left on your spoon then your curd is ready.
Remove the orange curd from the heat and strain into a large measuring jug so the curd is very smooth.
Drop the cold cubed butter into the curd and blend with an immersion (handheld) blender until glossy and smooth.
Place the measuring jug in a large saucepan and fill the sides with ice and cold water to create an ice bath. Allow the curd to cool in the measuring jug set in the ice bath for 15 minutes.
Pour the curd into sterilised jars, seal and place in the fridge to cool and set overnight.