Preheat the oven to 220°C.
Trim, peel, and cut the rhubarb into chunks then place in a pan over medium heat.
Add muscovado sugar, orange juice, and cinnamon and simmer for 8-10 minutes until tender.
Sieve the flour and mixed spice into a mixing bowl.
Rub the butter into the flour until the mixture resembles fine breadcrumbs.
Stir in the caster sugar.
Transfer the rhubarb to an ovenproof dish and cover with an even layer of crumble.
Bake for 10-12 minutes until the topping is golden-brown.
Heat the vanilla custard in a small pan and pour into a jug.
Serve the hot crumble with the prepared custard.