Preheat the oven to 200°C.
Grease an ovenproof dish with butter.
Pulse butter, sugar, biscuits, pistachios, and flour in a food processor until breadcrumb texture forms.
Layer forest fruits in the dish and cover with the crumble mixture.
Bake for 10 to 15 minutes until the topping is crisp.
Heat milk and cream in a saucepan until steaming, then cool slightly.
Whisk egg yolk and sugar together in a bowl.
Pour the warm milk mixture into the eggs while whisking constantly.
Simmer the mixture in a saucepan until the custard thickens.
Strain the custard through a sieve into a jug.
Serve the crumble topped with warm custard.