Preheat your oven to 350°F (175°C). Line an 8×8 inch baking pan with parchment paper, leaving an overhang on two sides for easy removal.
In a medium bowl, whisk together the gluten-free flour blend, almond flour, 1/2 cup sugar, and salt for the crust.
Cut in the cold butter using a pastry blender or your fingers until the mixture resembles coarse crumbs.
Mix the egg yolk and cold water into the flour mixture until a dough just comes together. Do not overmix.
Press two-thirds of the dough evenly into the bottom of the prepared pan to form the crust.
In a separate bowl, combine the chopped rhubarb, strawberries, 1/2 cup sugar, cornstarch, vanilla extract, and lemon juice. Mix gently until the fruit is coated.
Spread the fruit filling evenly over the crust layer.
Crumble the remaining one-third of the dough mixture over the top of the fruit filling.
Bake for 45 to 55 minutes, or until the topping is golden brown and the filling is bubbly.
Let the bars cool completely in the pan on a wire rack before lifting them out using the parchment paper overhang. Cut into squares.
Notes
For a chewier texture, use oat flour instead of the gluten-free all-purpose blend for the crust.
If your gluten-free flour blend does not contain xanthan gum, add 1/2 teaspoon to the dry ingredients.
These bars taste best after chilling for at least one hour before slicing.