Peel, quarter and core the apples. Slice into a medium-sized pan and add the water. Set over a medium heat and cook for about 10 minutes, stirring gently until the slices are just tender. Stir in the sugar then spoon into a heatproof serving dish.
Heat the grill. Meanwhile, crush the biscuits and combine with the chopped nuts. Scatter over the hot apples then quickly flash under the grill for a few seconds until lightly browned.
Serve immediately with warm custard, yogurt or ice cream.