1. Preheat oven to 350°F and line a 9-inch square baking pan with parchment paper, leaving overhang for easy removal.

2. Combine rhubarb, sugar, lemon juice, and cornstarch in a saucepan. Cook over medium heat, stirring frequently, until the rhubarb softens and the mixture thickens, about 8–10 minutes. Set aside to cool.

3. In a large bowl, mix flour, oats, brown sugar, and salt. Rub in the cold butter with fingertips or a pastry cutter until the mixture resembles coarse crumbs.

4. Press two-thirds of the crumble mixture firmly into the bottom of the prepared pan to form an even base. Reserve the remaining crumble for topping.

5. Bake the base for 15 minutes until lightly golden, then remove from the oven and allow it to cool slightly.

6. Heat the milk in a saucepan until steaming but not boiling. In a bowl, whisk egg yolks, sugar, and cornstarch until smooth. Gradually whisk the hot milk into the egg mixture, then return it to the saucepan and cook over medium heat, stirring constantly, until thickened, about 2–3 minutes. Stir in vanilla extract.

7. Spread the cooled rhubarb filling evenly over the baked base.

8. Pour the custard over the rhubarb layer and spread gently to cover.

9. Sprinkle the reserved crumble mixture evenly over the custard layer.

10. Bake for about 30 minutes until the crumble is golden and the custard is just set.

11. Allow the bars to cool completely in the pan, then refrigerate for at least 2 hours before lifting out and slicing into 16 bars.