Put blueberries and rhubarb chunks and lemon juice into a large, stainless steel pot, over medium heat
Cook, stirring occasionally until rhubarb is softened.
In a small bowl, whisk sugar and cornstarch together
Add to fruit mixture, stirring constantly until thickened and smooth.
Remove from heat.
Heat oven to 350°. Prepare a 9 x 13 baking pan by spraying with cooking oil, or, lining with parchment paper.
In a bowl, combine oats, flour, brown sugar, baking soda and salt.
Using a pastry blender to cut butter into flour mixture until it resembles course crumbs
Press half the flour mixture into prepared pan to form crust
Spread blueberry rhubarb mixture over the bottom crust
Spread remaining flour mixture over the fruit layer, pressing lightly
Bake in center of oven until golden, about 45 minutes
Cool completely on a wire rack before slicing into 24 squares
Notes
Don't have fresh blueberries? An equal quantity of thawed frozen blueberries (unsweetened) will work fine in this recipe.It's important to allow the bars to cool completely before slicing, so that they will hold their shape.