Preheat the oven to 350 degrees F. Lightly spray a 9" x 9" or 8" x 10" baking dish with cooking spray. Line with foil for easily lifting the bars out of the pan (optional).
Mix together the flour, oats, brown sugar, baking powder, and salt.
With a pastry cutter or a fork, or your hands, cut the butter into the oatmeal blend until it resembles coarse crumbs. Pour a little more than half of the mixture into the pan and pat lightly with your fingertips or a fork to pack it down to form the cookie base.
Spoon the strawberry rhubarb over the surface of the cookie and use the back of your spoon to spread it evenly around.
Top with the remaining oatmeal mixture.
Bake until light golden brown on top, about 30 to 40 minutes.
Remove from the oven and let cool completely in pan.
When cool, use the aluminum foil to easily lift the entire cookie out of the pan. Peel the foil from the base and cut into squares or triangles.
Nutritional information is provided as a courtesy and is sourced from the USDA Food Database.