Preheat oven to 350F (180C). Prepare a 9 x 13″ baking pan.
Combine rhubarb strawberries and lemon juice in a saucepan, cover and cook over medium heat, stirring occasionally until fruit is tender, about 12-15 minutes.
Combine sugar, and cornstarch and stir into fruit mixture and continue cooking, stirring constantly, until mixture comes to a boil and thickens. Remove from heat and set aside.
Combine crust ingredients and mix with hands or blender on low until fully mixed and resembles coarse crumbs. Reserve 1 cup for top of bars. Spread remaining in baking pan and press down.
Spread fruit filling over top and smooth out. Sprinkle reserved topping over top of fruit mixture and bake for 30-35 mins. or until top is golden brown. You will be tempted to cut it right away but wait until it has cooled thoroughly to slice. Trust us!
Yield will depend on how large you cut your squares.