Preheat the oven to 200C/400F/Gas 6.
For the crumble filling, heat the butter in a saucepan, add the apples, sugar and lemon thyme and cook over a medium heat, stirring from time to time, for 5-8 minutes, or until tender. Grease an ovenproof dish and pour the mixture into the dish.
For the crumble topping, rub the butter and flour together in a large bowl until the mixture resembles breadcrumbs. Add the sugar and mix well, then spoon the crumble topping over the apple mixture.
Transfer to the oven and bake for 15-20 minutes, or until golden and bubbling.
Meanwhile, for the custard, place the milk, cream and thyme into a saucepan and bring to a simmer. Simmer very gently for five minutes to infuse the flavour of the thyme.
Place the egg yolks and sugar into a bowl and whisk together. Pour the warm milk onto the egg mixture and stir well, then return the mixture to the pan and cook gently until just thickened.
To serve, place a spoonful of crumble onto each of six plates and pour over the thyme custard.