Melt the butter in a pan, then add the pears, rhubarb, sugar, cinnamon and cloves, and cook over a low heat for 10-12 mins or until just tender. Divide the rhubarb between 4 ovenproof dishes (or use 1 large dish) and set aside.
Heat oven to 200C/180C fan/gas 6. To make the crumble topping, tip all the ingredients into a food processor and pulse to crumbs. Sprinkle the topping over the fruit filling, then bake for 30 mins or until golden brown on top. Serve with vanilla ice cream or double cream.