Preheat the oven to 220C/425F/Gas 7.
Trim and peel the rhubarb. Cut the rhubarb into chunks and place into a pan over a medium heat.
Add the light muscovado sugar, orange juice and cinnamon. Bring to the boil, then reduce the heat to simmer for 8-10 minutes, until the rhubarb is tender but still holding its shape.
Meanwhile, sieve the flour and mixed spice into a bowl.
Add the butter and rub into the flour mixture with your fingers until the mixture resembles fine breadcrumbs.
Add the caster sugar and stir to combine.
Transfer the stewed rhubarb to an ovenproof dish and sprinkle over a thin, even layer of the crumble topping.
Transfer the crumble to the oven to bake for 10-12 minutes, or until the topping is cooked through and lightly golden-brown.
Heat the vanilla custard in a small pan, then transfer to a serving jug.
Serve the crumble in its dish at the table (place a thick heatproof board underneath), with a jug of the custard so that guests can help themselves.