Preheat the oven to 200°C.
Cook the butter, apples, sugar, and lemon thyme in a pan for 8 minutes until tender.
Transfer the apple mixture into a greased ovenproof dish.
Rub the butter and flour together until the mixture resembles breadcrumbs then stir in the sugar.
Spoon the crumble topping over the apples and bake for 20 minutes until golden.
Simmer the milk, cream, and thyme sprigs in a pan for 5 minutes.
Whisk the egg yolks and sugar together in a separate bowl.
Pour the warm milk over the egg mixture while stirring well.
Return the custard to the pan and cook gently until thickened.
Plate the crumble and pour the warm thyme custard over the top.