Wash rhubarb well and cut into 3-4cm lengths. If using the apple and pear, halve them, remove the cores, then cut into slices. Put the fruit into a greased 2-litre (8-cup) pudding basin, along with the sugar and lemon juice. Toss well and then pour the orange juice over the top. To make the topping, put the butter and flour into a food processor and process until it is a bread crumb consistency. Transfer to a bowl and add the brown sugar, rolled oats and if using, the walnuts. Scatter evenly over the rhubarb. Bake in preheated 180 C oven for 25-40 minutes, or until golden on top and the fruit is tender. Sprinkle with icing sugar and serve.