Preheat the oven to 375°F. Spray a 9x13 inch pan with cooking spray. Line the pan with parchment paper, leaving 1 inch of extra parchment paper hanging over the sides.

In a large bowl, combine the flour, sugar, baking powder, cinnamon, and salt. Cut the cold butter into ½ inch chunks over the dry ingredients. Using your fingers, squish the butter and dry ingredients together until the mixture is crumbly.

In a small bowl, toss egg with a fork, then massage into the crumbly dry mixture with your hands. The dough will be crumbly.

Scoop 1 cup of the crumbly mixture and set it aside.

Place the remaining crumbly mixture into the bottom of the lined pan and press it down firmly with your hands, to form an even crust layer of dough at the bottom of the pan.

In a large bowl, combine the rhubarb, strawberries, sugar, cornstarch and lemon juice. Gently mix it with a spatula. Spread the fruit filling evenly over the bottom crust.

Sprinkle the reserved crumbly mixture over the fruit filling. Bake in the oven 40-45 minutes or until the top of the fruit is bubbling and the crumb topping is golden brown.

Cool completely in the pan. Once the dessert has reached room temperature, use the 1 inch extra parchment paper to lift up and move the entire block carefully to a cutting board. Cut into bars. Serve at room temperature. Store in the refrigerator in an airtight container up to 3 days.