For the crumb layer:
Preheat the oven to 350 degrees and line a 9x9 baking dish with parchment paper. Spray pan and parchment paper with non-stick cooking spray.
In a large bowl whisk together the dry cookie mix, cinnamon, brown sugar and oatmeal. Whisk the vanilla into the melted butter, then pour over oatmeal mixture.
Stir with a spatula or wooden spoon until the butter is evenly distributed. Press 2/3 of the mixture into the prepared baking dish, set aside the remaining 1/3 of the mixture.
For the filling:
In a large bowl, combine diced strawberries and rhubarb and toss together with lemon juice. Combine cane sugar and cornstarch in a small bowl. Pour sugar mixture over the strawberries and rhubarb and toss to coat evenly.
Pour the strawberry mixture over the bottom dough layer in the baking dish. Sprinkle the remaining 1/3 crumb mixture evenly over the top. Bake 45-50 minutes until the top is golden brown and crisp.
Cool on a wire rack until room temperature then cut into squares. If desired, serve with vanilla ice cream!