Preheat your oven to 350°F (175°C). Grease a 9x9-inch baking pan or line it with parchment paper for easy removal.

In a medium bowl, combine the sliced strawberries, chopped rhubarb, ½ cup of granulated sugar, cornstarch, lemon juice, and vanilla extract. Mix well and set aside to let the juices meld.

In a separate bowl, whisk together the flour, baking powder, and salt.

In another bowl, cream the softened butter with the remaining ½ cup of granulated sugar and brown sugar until light and fluffy.

Gradually add the flour mixture to the butter mixture, stirring until crumbly. Stir in the rolled oats until evenly combined.

Press about two-thirds of the crumble mixture into the bottom of the prepared baking pan to form the base.

Spread the strawberry-rhubarb filling evenly over the crust.

Sprinkle the remaining crumble mixture on top of the filling.

Bake in the preheated oven for 35-40 minutes, or until the top is golden brown and the filling is bubbling.

Allow to cool in the pan for at least 30 minutes before cutting into bars.