Heat the raspberries, water and lemon juice in a pan until the raspberries start to break down.
Blend the raspberry mixture and pass through a sieve to remove all the pips. This takes a while and you have to really push the liquid through the sieve. I find it helps to do it in batches and scrape the seeds out of the sieve between each batch as they will start to clog up the holes. You should be left with about 200-250ml juice.
Whisk 150g caster sugar with 3 eggs in a microwave safe bowl. This amount of sugar makes quite a sweet curd, you can add a bit less if you prefer.
Mix in the raspberry juice. Cut the butter into small cubes and add this as well.
Put in the microwave and heat for 40 seconds. Whisk well. Repeat until the mixture has become thick and silky. Takes 5 to 10 minutes depending on your microwave power.
I find that the lovely red colour of the raspberries gets lost when they are put through the sieve and they go a dull mauve colour. To brighten up the finished curd I whisk in a bit of claret gel food colour.