1.Preheat oven to 200°C (180°C fan-forced). Grease six ½-cup (125ml) ovenproof dishes.
2.Stir cornflour and sugar with rind, juice and the water in medium heatproof bowl over medium saucepan of simmering water, stir in eggs and butter until mixture is thickened. Remove from heat, stir in sour cream. Pour custard into dishes.
3.Make crumble topping, Sift flour into medium bowl, stir in oats, rub in butter. Stir in remaining ingredients, sprinkle topping over custard.
4.Bake crumble about 20 minutes or until topping is crisp.