Step 1: Preheat your oven to 350°F (175°C) and grease a 9×9 inch baking pan or line it with parchment paper.
Step 2: In a large bowl, whisk together the flour, 1/2 cup of sugar, and salt.
Step 3: Cut in the cold cubed butter using a pastry cutter or your fingers until the mixture resembles coarse crumbs.
Step 4: Press two-thirds of the crumble mixture firmly into the bottom of the prepared baking pan.
Step 5: Bake the crust for 12-15 minutes until it is lightly set but not browned.
Step 6: While the crust bakes, combine the rhubarb, strawberries, remaining 1/2 cup of sugar, cornstarch, and lemon juice in a medium bowl; toss to coat.
Step 7: Spread the fruit mixture evenly over the par-baked crust.
Step 8: Sprinkle the remaining crumble mixture over the top of the fruit layer.
Step 9: Bake for an additional 30-35 minutes until the topping is golden brown and the fruit filling is bubbling.
Step 10: Allow the bars to cool completely in the pan on a wire rack before slicing into squares.