Preheat oven to 350ºF.

Combine rhubarb, blueberries and lemon juice in a large saucepan and cook over medium heat until rhubarb is softened. No added liquid is needed as the water from the rhubarb and blueberries will come out naturally.

In a small bowl, combine sugar and cornstarch. Mix well to break up the lumps of cornstarch. Set aside.

While rhubarb and blueberries are cooking, begin making the crust. Combine flour, oats, brown sugar, butter, baking soda and salt together in a bowl until well mixed and crumbly. Reserve 1 1/2 cups of crust to be used as a topping. Grease a 13×9 pan with cooking spray. Press remaining crust mixture into the pan in an even layer. Set aside.

When rhubarb and blueberries are softened, use a potato masher to mash and smooth out the larger chunks. Once bigger pieces are broken up, mix in the sugar/cornstarch mixture. Continue to stir until thickened.

Once mixture is thickened, pour over crust. Top with reserved crust mixture.

Bake at 350ºF for 30 minutes.

Tips:
1. Mixing the sugar and cornstarch together will prevent lumps of cornstarch in your mixture.
2. The bars will puff up considerably, so don’t be alarmed when you open the oven to take out the bars. They will be a bit jiggly. That’s OK! They are done!
3. When they cool, they’ll “deflate” a bit and set up nicely.