Place bread in a buttered 2-litre (8-cup) ovenproof dish. Combine the milk and butter in a saucepan then bring to the boil. Remove from the heat.
Whisk the eggs, sugar and cinnamon in a bowl until combined. Gradually whisk in the hot milk mixture. Add the rhubarb. Pour over the bread cubes and stand for 15 minutes. Sprinkle walnuts over the top.
Bake for 50 minutes or until lightly browned and the custard is just set. Stand for 10 minutes before serving.