Preheat oven to 350°F (175°C). Line an 8×8-inch or 9×9-inch baking pan with parchment paper, leaving overhang on two sides for easy removal. Lightly grease the parchment.
In a medium bowl, whisk flour, sugar, and salt. Add cold butter cubes. Use a pastry cutter or your fingers to work the butter in until the mixture looks like coarse crumbs with some pea-sized pieces. Press the mixture firmly and evenly into the bottom of the prepared pan. Bake for 12–14 minutes until the edges just start to turn light golden. Remove and set aside.
While the crust bakes, combine chopped strawberries, sugar, cornstarch, lemon juice, and vanilla in a medium saucepan. Cook over medium heat, stirring often, for 6–8 minutes. The berries will release juice, then the mixture will thicken and turn glossy. It should coat the back of a spoon. Remove from heat.
In a clean bowl, mix flour, oats, brown sugar, and salt. Add cold butter and rub it in with your fingertips until large, irregular clumps form. Do not overwork keep some bigger pieces for texture.
Spread the warm strawberry filling evenly over the partially baked crust. Sprinkle the crumb topping evenly over the fruit. Bake for 25–28 minutes until the topping is golden and the filling bubbles gently around the edges.
Place the pan on a wire rack. Let the bars cool fully at room temperature, then refrigerate for at least 1–2 hours. This sets the filling so the bars cut cleanly. Use the parchment to lift them out, then slice into squares or rectangles with a sharp knife.
