Preheat oven to 180C. Combine the flour, sugar, butter and oats in a big bowl.

Use your fingertips to rub the butter into the flour mixture until the mixture resembles fine breadcrumbs. Stir well in the walnuts.

Spoon the stewed rhubarb and apple into a ovenproof dish. Scatter the walnut mixture evenly over the stewed rhubarb and apples. Bake in oven for 20-25 minutes or until golden.