In a bowl set over simmering water combine eggs, sugar, juice of both lemon and mandarin.
Stir until the the sugar has dissolved.
Add the butter and the thyme sprigs and continue to stir until the butter has melted and the mixture has thickened to cover the back of a spoon.
Remove the sprigs of thyme.
Pour the contents into sterilised jars, seal and allow to cool. Will keep in a dark cool spot for up to a year. Once opened keep refridgerated and use within 2 weeks.