Buttery shortbread crust, jammy strawberry-rhubarb filling, and a golden crumble top—these bars are bright, sweet-tart, and perfect for spring gatherings.
Preheat the oven to 350°F. Line a 9x13 inch baking pan with parchment paper, leaving overhang on two sides for easy lifting.
Make the crust: In a large bowl whisk together flour, 0.5 cup sugar, and 0.5 tsp salt. Stir in melted butter and 1 tsp vanilla until a soft, sandy dough forms.
Reserve about 1.5 cups of the crumb mixture for topping. Press the remaining dough firmly and evenly into the prepared pan.
Par-bake the crust for 10 minutes. While it bakes, prepare the filling.
Make the filling: In a bowl combine rhubarb, strawberries, 0.75 cup sugar, cornstarch, lemon juice, lemon zest, 1 tsp vanilla, and 0.25 tsp salt. Toss until the fruit is evenly coated and glossy.
Spread the fruit mixture over the warm crust in an even layer. Sprinkle the reserved crumb mixture evenly over the top.
Bake for 35 to 40 minutes, until the topping is golden and the filling is bubbling around the edges.
Cool the pan on a rack until completely set, about 2 hours. Lift out using the parchment and cut into 16 bars.
Notes
For cleaner slices, refrigerate for 45 minutes after cooling, then cut. If your fruit is very juicy, add an extra 1 tbsp cornstarch. Bars keep in an airtight container at room temperature for 2 days or refrigerated for 5 days.