Step-by-Step Instructions:

Prep the Pan: Preheat oven to 350°F (177°C). Line a 9×13-inch baking pan with parchment paper, leaving extra overhang on the sides for easy removal later.

Make the Crust and Crumb Topping Base: In a large mixing bowl, whisk together the flour, granulated sugar, brown sugar, baking powder, cinnamon (if using), and salt. Add the cold, cubed butter. Use a pastry cutter, two forks, or a food processor to cut in the butter until the mixture resembles coarse crumbs—this should take about 5 minutes by hand.

Add Wet Ingredients: In a small bowl, whisk together the egg, milk, and vanilla extract. Pour this mixture into the flour/butter mixture. Stir gently, using a spoon or your hands, until the mixture resembles moist, crumbly sand.

Form the Crust: You should have about 6 cups of mixture. Set aside 2 cups for the topping. Press the remaining 4 cups evenly into the bottom of the prepared pan using your hands or a flat spatula. It’s okay if the mixture feels crumbly—it will firm up when baked.

Prepare the Filling: In a medium bowl, gently toss the chopped strawberries, sliced rhubarb, sugar, cornstarch, and orange zest together. Stir until evenly coated.

Layer the Filling: Spread the fruit filling evenly over the crust layer.

Add the Topping: Sprinkle the reserved 2 cups of crumb mixture evenly over the fruit filling. Then sprinkle the oats over the top for texture and appearance. Press down lightly with a spatula to help the crumb topping adhere.

Bake: Bake for 42–50 minutes or until the top is golden brown and a toothpick inserted comes out mostly clean with a few fruity bits. Start checking at 42 minutes; mine are usually perfect around 45.

Cool: Let the bars cool completely in the pan on a wire rack—this may take 1–2 hours. For quicker cooling, refrigerate the pan after about an hour.

Cut and Serve: Once fully cooled, use the parchment overhang to lift the bars out of the pan. Cut into squares and serve.