Method
Grease a 2-litre baking dish with butter. Butter one side of each bread slice, then cut each slice into 2 triangles.
Arrange the triangles butter side up, slightly overlapping like roof tiles, tucking the sultanas between the layers so none sit on top.
In a bowl, whisk the eggs and caster sugar until smooth, then whisk in the milk, cream, vanilla, lemon zest and half the nutmeg.
Pour the custard slowly over the bread, pressing the slices down gently with a spatula. Leave to soak for 30 minutes at room temperature.
Heat the oven to 180°C (160°C fan) / 350°F. Sprinkle the demerara sugar and remaining nutmeg over the top.
Bake for 35-40 minutes, until puffed and golden with crisp edges and only a slight wobble in the centre (71°C / 160°F inside).
Rest for 10 minutes, then serve warm with pouring custard, cream or vanilla ice cream.