Add strawberries, sugar, and lemon juice to a saucepan over medium heat. Cook, stirring occasionally, for 8–10 minutes until the strawberries release their juices and begin to break down.

In a small bowl, stir together cornstarch and water. Add to the strawberry mixture and cook for 2–3 more minutes until thickened and glossy. Remove from heat and let cool completely.

Preheat oven to 350°F. Line a 9x13 baking dish with parchment paper.

In a large bowl, whisk together flour, granulated sugar, brown sugar, baking powder, and salt.

Add melted butter, egg, and vanilla extract. Mix until a soft, crumbly dough forms.

Press about two-thirds of the dough firmly into the bottom of the prepared pan.

Spread the cooled strawberry filling evenly over the crust.

Pinch the remaining dough into chunks and scatter over the top to create the crumble layer.

Bake for 40–45 minutes, until the top is golden and the edges are set.

Let cool at room temperature, then refrigerate for 1–2 hours before slicing for clean bars.