1 Preheat oven to 350°F (175°C). Line a 9×13-inch (23×33 cm) baking pan with parchment, leaving an overhang to lift the bars out.

2 Make the crust: In a bowl combine flour, 1/2 cup sugar and salt. Cut in cold butter with a pastry cutter or fork until mixture resembles coarse crumbs. Stir in oats if using.

3 Press two-thirds of the crust mixture firmly and evenly into the prepared pan to form a compact base. Reserve the remaining crumb for topping.

4 Blind-bake the crust for 12–15 minutes until just set and very lightly golden. Remove from oven and reduce oven temperature slightly if needed.

5 Prepare the rhubarb: Toss chopped rhubarb with 1/2 cup sugar, 1 tbsp lemon juice and 1 tbsp cornstarch. Let sit 5 minutes, then cook briefly in a small saucepan over medium heat for 3–5 minutes until it softens and releases juices but still holds some shape. Remove from heat and cool slightly.

6 Make the custard: Whisk eggs with 3/4 cup sugar until smooth. Whisk in 2 tbsp cornstarch until dissolved. Gradually whisk in milk and vanilla until combined.

7 Pour the custard mixture over the par-baked crust, smoothing gently. Spoon the cooled rhubarb mixture evenly over the custard layer. Sprinkle the reserved crumb mixture on top, pressing lightly.

8 Bake for 25–30 minutes more, until the custard is set (it should jiggle slightly in the center but not be liquid) and the top is golden.

9 Cool completely in the pan on a wire rack. For clean slices chill in the refrigerator for at least 1–2 hours.

10 Lift the slab from the pan using the parchment overhang. Dust with powdered sugar if desired, then cut into 12 bars and serve.