1. Preheat the oven to 200°C/400°F/ Gas 6. Place the gooseberries in a pan with the caster sugar. Cover the pan and cook over a low heat for 10 minutes, until the gooseberries are Just tender. Tip into an ovenproof dish.
2. To make the crumble, place the oats, flour and oil in a bowl and stir with a fork until evenly mixed.
3. Stir in the demerara sugar and walnuts, then spread evenly over the gooseberries. Bake for 25-30 minutes, or until golden and bubbling. Serve hot with yogurt, or custard made with skimmed milk.