Place the white chocolate into a plastic microwave safe bowl and pour the lemon juice over the top.
Put the bowl in the microwave and heat for 20 seconds (at 1000W).
Remove the bowl from the microwave and give the mixture a good stir to help melt the chocolate. As the chocolate melts it should combine with the lemon juice and make a smooth ganache.
If the chocolate has not yet fully melted, or you are finding it hard to mix it into the lemon juice, return the bowl to the microwave and heat for a further 10 seconds, then stir again.