Preheat the oven to 200C/400F/Gas 6.
Grease an ovenproof dish with butter.
Place the butter, sugar, ginger biscuits, pistachios and flour into a food processor and pulse until the mixture resembles breadcrumbs.
Place a layer of the frozen forest fruits into the base of the ovenproof dish and top with the crumble mixture.
Transfer to the oven and bake for 10-15 minutes, until the topping is crisp and golden.
For the custard, place the milk and cream into a saucepan over a medium heat and heat until steaming. Remove from the heat and allow to cool slightly.
Whisk the egg yolk and sugar together in a bowl.
Gradually pour the milk and cream mixture into the yolk and sugar, whisking constantly.
Return the custard mixture to the milk saucepan and simmer gently over a medium heat, stirring constantly, until the custard thickens and coats the back of a spoon.
Strain the custard through a sieve into a jug.
To serve, spoon a portion of the crumble into a bowl and pour over some custard.