Whisk the eggs and milk together in a bowl. Place the brioche or panettone in a shallow dish and pour the milk mixture over the top. Allow the milk to soak in thoroughly, then turn the bread over and soak the other side — if you are using panettone it will need to be soaked for a little longer because it is drier.
Heat a large non-stick frying pan over medium–high heat and melt half the butter. Add four slices of bread to the pan and fry for about 1 minute, until golden. Turn over and cook until the other side is golden. Repeat with the remaining bread.
Meanwhile, to make the berry sauce, put half the berries in a blender with the sugar and lemon juice and purée until smooth. Pour into a bowl, add the remaining berries and stir to combine.
Serve the French toast immediately with the berry sauce and a sprinkling of icing sugar.