Preheat the oven to 180C (160C fan) and grease a shallow baking dish roughly 20cm by 25cm.
Spread each slice of bread with butter, then spread jam on top. Cut each slice in half diagonally.
Arrange the bread triangles in the dish, overlapping them slightly so the jammy sides are visible.
Whisk the milk, eggs, caster sugar and vanilla together in a jug until smooth.
Pour the custard mixture evenly over the bread, then sprinkle the top with ground nutmeg if using.
Leave to stand for 10 minutes so the bread soaks up the custard, then bake for 30 to 35 minutes until golden and just set.
Serve warm, on its own or with a splash of cold milk.