Preheat the oven to 180°C/gas mark 4. Mix the oats, caster sugar, butter, flour and vanilla extract together, rubbing the butter between your fingertips until the mixture has a crumbly texture.
Mix the rhubarb and forest fruits together in a bowl with the lime and cinnamon. Divide into 4 large ramekins and sprinkle with the crumble topping.
Bake for 25-30 minutes until golden brown. Serve with custard or double cream.