Preheat oven to 350°F (177°C). Line the bottom and sides of a 9×13-inch baking pan with parchment paper, leaving an overhang on the sides to lift the finished bars out (makes cutting easier!). Set aside.

Whisk the flour, sugar, baking powder, cinnamon, and salt together in a large bowl. Add the cubed butter and using a pastry cutter, two forks, or a food processor, cut in the butter until all the flour is coated and resembles pea-sized crumbles. (See photo above for a visual.) This takes at least 5 minutes of cutting in with a pastry cutter.

Whisk the egg, milk, and vanilla together in a small bowl. Pour over the flour/butter mixture and gently mix together until the mixture resembles moist crumbly sand. Use your hands if needed—the mixture comes together easier with your hands than with a spoon. See photo above for a visual.

You will have about 6 cups of the crust/crumble mixture. Set 2 cups aside. Pour the remaining into the prepared pan and flatten down with your hands or a flat spatula to form an even crust. It will be a little crumbly—that’s ok. Set aside. (Oats will be used in the topping in the next step.)

Gently mix all of the filling ingredients together. Spread over the crust. Sprinkle the remaining crumble mixture all over the filling. Sprinkle the oats on over top. With the back of a large spoon or flat spatula, lightly press the topping down so it’s a bit snug on the strawberry rhubarb layer.

Bake for about 42-50 minutes or until the top is lightly browned and a toothpick comes out *mostly* clean (with a few jammy strawberry/rhubarb specks!). Mine take about 45 minutes. Remove from the oven and allow the bars to cool completely in the pan set on a wire rack. After about 1 hour, I stick the whole pan in the refrigerator to help speed things up.

Lift the cooled bars out using the parchment paper overhang on the sides. Cut into squares. Cover and store leftover strawberry rhubarb bars at room temperature for up to 2 days or in the refrigerator for up to 1 week.