Prepare the crumble. In a bowl, combine the flour, oats, sugar, chopped walnuts, and salt. Add the cold butter, cut into cubes, and rub everything together with your fingertips until crumbly. Place the crumble in the fridge while preparing the filling.

Preheat the oven to 180°C.

Cut the rhubarb into pieces and peel and dice the apples. Transfer the fruit to a baking dish and mix with sugar and potato starch.

Sprinkle the chilled crumble evenly over the fruit. Bake for about 35–40 minutes, until golden brown on top.

Meanwhile, prepare the custard sauce. Heat the milk, cream, vanilla paste, and a pinch of salt. In a separate bowl, whisk the egg yolks with sugar, then slowly pour in a little of the hot milk mixture while whisking constantly to temper the yolks. Pour everything back into the saucepan and cook over low heat, stirring continuously, until the sauce slightly thickens. Do not let it boil. For an extra smooth sauce, strain it through a sieve before serving.

Serve the rhubarb crumble warm with the vanilla custard sauce.