Follow these steps carefully for perfect Strawberry Oat Crumble Bars every time:

1. Preheat and Prep: Preheat your oven to 375°F (190°C). Grease a 9×13 inch baking pan thoroughly with butter or non-stick spray. For easier removal and cleanup, line the pan with parchment paper, leaving a slight overhang on two opposite sides to act as handles. Grease the parchment paper as well.
2. Make the Crumble Mixture: In a large bowl, whisk together the all-purpose flour, rolled oats, packed light brown sugar, baking soda, and salt. Ensure there are no large clumps of brown sugar.
3. Cut in the Butter: Add the cold, cubed unsalted butter to the dry ingredients. Using a pastry blender, two knives, or your fingertips, cut the butter into the flour mixture until it resembles coarse crumbs with some pea-sized pieces of butter still visible. Be careful not to overmix or let the butter get too warm; you want distinct pieces of butter coated in flour, not a uniform paste.
4. Press the Base: Reserve approximately 1 ½ cups of the crumble mixture for the topping and set it aside. Take the remaining crumble mixture and press it firmly and evenly into the bottom of the prepared 9×13 inch baking pan. Use the bottom of a measuring cup or glass to help create a compact, level base.
5. Prepare the Strawberry Filling: In a separate medium bowl, gently toss the chopped fresh strawberries with the granulated sugar, cornstarch, fresh lemon juice, and vanilla extract. Stir until the strawberries are evenly coated and the cornstarch is mostly dissolved. Let it sit for about 5 minutes; the strawberries will start releasing some juices, creating a syrupy mixture.
6. Assemble the Bars: Pour the prepared strawberry filling evenly over the pressed crumble base in the baking pan. Spread it out gently with a spatula, ensuring it reaches the edges.
7. Add the Topping: Sprinkle the reserved 1 ½ cups of crumble mixture evenly over the strawberry layer. Gently pat it down slightly, but don’t press firmly like the base.
8. Bake: Place the baking pan in the preheated oven. Bake for 35-45 minutes, or until the crumble topping is golden brown and the strawberry filling is bubbly around the edges. The centre should look set.
9. Cool Completely: This step is crucial! Remove the pan from the oven and place it on a wire rack. Let the bars cool completely in the pan before attempting to cut them. This usually takes at least 2-3 hours at room temperature, or you can speed it up slightly in the refrigerator after the first hour of cooling. Cooling allows the filling to set properly and prevents the bars from falling apart when cut.
10. Cut and Serve: Once completely cooled, use the parchment paper overhangs to lift the entire slab out of the pan onto a cutting board. Use a sharp knife to cut into squares or bars (e.g., 16 squares). Serve and enjoy!