In a food processor, purée 250ml (1 cup) strawberries, sugar, water and cornstarch until smooth.
In a saucepan, bring the purée to a boil, stirring constantly. Add the remaining strawberries. Bring to a boil and cook for about 1 minute. Pour into a bowl and let cool. Cover and refrigerate until the mixture has chilled.
In a small sundae dish, place a scoop of ice cream. Spoon strawberry topping. With a pastry bag fitted with a star tip, garnish with a rosette of whipped cream. Sprinkle with crumble.