Pulse all the pastry ingredients in a food processor then wrap in clingfilm and chill for 30 minutes. Roll the pastry out and use to line a 23cm tart tin. Chill for 30 minutes.
Heat the oven to 180C/fan 160C/gas 4. Line the tart with baking paper, fill with baking beans and blind bake for 20 minutes. Remove the beans and paper and brush with beaten egg. Cook for 5 minutes.
Put the rhubarb, spices, sugar and orange in a pan and cook gently until just soft. Cool a little, then spoon into the pastry case. Rub flour and butter together to make breadcrumbs, stir in the oats, sugar and cinnamon, then spoon on top of the fruit. Bake for 20-25 minutes or until the top is crisp and golden. Serve with custard.