Preheat oven to 350°F (175°C). Line a 9x13 inch baking pan with parchment paper, leaving an overhang on the sides for easy lifting.

Prepare the Crust/Crumble: In a large bowl, whisk together the flour and salt. Cut in the cold butter using a pastry blender or your fingers until the mixture resembles coarse crumbs. Stir in 1 cup of sugar and the vanilla extract until just combined.

Press two-thirds (about 2 cups) of the mixture evenly into the bottom of the prepared pan to form the crust. Reserve the remaining one-third for the topping.

Prepare the Filling: In a separate bowl, gently toss the chopped rhubarb and strawberries with 3/4 cup sugar, cornstarch, and lemon juice until evenly coated. Let stand for 5 minutes.

Spoon the fruit filling evenly over the shortbread crust in the pan. Sprinkle the reserved one-third of the crust mixture evenly over the top of the fruit filling to create the crumble layer.

Bake for 45 to 50 minutes, or until the topping is golden brown and the filling is bubbly. If the topping starts to brown too quickly, loosely cover the pan with foil.

Allow the bars to cool completely in the pan on a wire rack for at least 2 hours before lifting them out using the parchment sling and cutting them into 16 squares.