Seville oranges have a limited season, just a few weeks of availability in January. I don't think you can get them in Australia, but in the UK they are usually available in the greengrocers, mostly for marmalade makers. Not being a fan of the preserve, I spritz it where I would lemons. You can get a very good approximation of the flavour by mixing regular orange juice with lime juice, to get the flavour of the orange with the correct eye-wincing sourness, if Seville oranges elude you.
Despite the winter associations of Seville oranges, this looks and tastes like summer in a jar. I'll post some recipes using the curd over the next few days, so watch this space.
4 large oranges, e.g. navel, zested and juiced
6 limes, zested and juiced
150g caster sugar
4 eggs, plus 2 egg yolks
150g butter, cubed
First sterilize your jar; wash with hot soapy water and let it dry out in a low oven.
Put the juices and zests in the saucepan with the eggs, egg yolks, and sugar. Beat together to make it a uniform consistency. Over a low heat, stir with a wooden spoon. Once it is heated through, add a cube of butter, stirring all the time until in is melted and mixed in. Continue to add the butter cubes, one by one, until they are all used. Keep cooking and stirring the curd. Once it thickly coats the back of the spoon, it is ready. Pour into your sterilised jar. Once it is cool, keep in the fridge for up to a week.