Heat oven to 200C/fan 180C/gas 6. For the topping, tip all the ingredients together and rub with your fingers until you have a rough pastry, then set aside in the fridge.
Peel the pears and chop into large chunks, then chop the rhubarb into finger-length batons. Tip into a large ovenproof dish and toss with the sugar and orange zest and juice. Crumble over the topping and bake for 40 mins until golden and bubbling at the sides. Leave to cool for 5 mins before spooning into bowls and serving with custard or ice cream.