Whisk the egg yolks and sugar together in a bowl until well blended.
Chop the fresh thyme finely and place into a pan, along with the milk and cream. Bring to the boil.
Once the milk and cream have reached boiling point, take off the heat. Pour a little of the milk and cream onto the eggs and sugar and mix well. Pour back into the pan. Return to the heat and, using a whisk, lightly stir to thicken. Do not boil.
As the egg yolks warm, the cream will thicken to a custard. Keep stirring with a wooden spoon until it coats the back of a spoon. Remove from the heat and pass through a sieve into a clean saucepan, discarding the thyme leaves. Leave to cool, stirring occasionally until it cools completely. The custard can be served cold or hot.