01 - Set the oven to 350°F (180°C) to prepare for baking.
02 - In a large bowl, gently toss mixed berries with sugar, cornstarch, lemon juice, and vanilla extract. Spread evenly in a 8x12 inch baking dish.
03 - Combine flour, oats, brown sugar, cinnamon, and salt in a separate bowl. Incorporate diced cold butter using fingertips until mixture resembles coarse crumbs.
04 - Evenly scatter the crumble topping over the berry filling in the baking dish.
05 - Place the dish in the oven and bake for 35 minutes or until topping is golden and fruit mixture is bubbling.
06 - While baking, heat milk and cream in a saucepan over medium heat until just steaming, avoiding boiling.
07 - Whisk together egg yolks, sugar, vanilla extract, and cornstarch in a bowl until smooth.
08 - Gradually pour the hot milk mixture into the egg mixture while whisking continuously to temper the eggs.
09 - Return combined mixture to saucepan and cook over medium-low heat, stirring constantly until thickened, about 5 minutes; do not boil.
10 - Remove from heat and strain custard into a jug to remove any lumps.
11 - Serve the warm berry crumble with a generous pour of fresh custard.