Preheat the oven to 170C/150C Fan/Gas 3½.
Remove any hard crusts from the bread, then butter each slice on one side. Cut the slices into triangles.
Peel and core the apple, then cut it into thin slices.
Arrange a layer of the bread triangles on the bottom of a 1.5-litre/2½-pint baking dish, butter side-down. Put a layer of the apple slices on top. Scatter over half the raisins.
Repeat with the remaining bread and apple, adding the raisins as you go. Finish with a layer of bread, butter side-up.
Beat together the milk, eggs and sugar in a jug until combined. Pour the mixture over the bread, pushing down the top layer of bread so it is submerged for the beginning of the soaking period. Leave to stand for at least 15 minutes and up to 1 hour.
Bake in the oven for 35-45 minutes, or until set with a golden top. Serve warm.