Cube the bread into bite-sized pieces, ideally using slightly stale or day-old bread to better absorb the custard mixture. Place the cubed bread and raisins (if using) into the slow cooker, spreading them evenly to ensure balanced flavor in every portion.

In a medium-sized mixing bowl, whisk together the milk, eggs, melted butter, sugar, vanilla extract, and nutmeg until the mixture is smooth and well-combined. This custard will provide the bread pudding with its rich, creamy texture.

Pour the custard mixture evenly over the bread and raisins in the slow cooker. Gently stir to coat each piece of bread with the mixture, ensuring the custard soaks into the bread for a uniform texture throughout the pudding.

Cover the slow cooker with its lid, and set it to cook on the Low setting for approximately 3 hours. After this time, check for doneness by inserting a knife into the center; if it comes out clean, the pudding is ready.

Once cooked, uncover the slow cooker and allow the pudding to cool slightly before serving. Serve warm, and if desired, garnish with a scoop of vanilla ice cream or a drizzle of caramel sauce for added indulgence.