Preheat your oven to 350°F. 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. Stir well and set aside to let the mixture macerate for about 15 minutes.
In another bowl, whisk together the flour, baking powder, and salt.
In a large mixing bowl, cream together the softened butter and remaining ½ cup of granulated sugar until light and fluffy. Beat in the egg until well combined.
Gradually add the flour mixture to the butter mixture, mixing until just combined. Stir in the rolled oats and brown sugar until evenly distributed.
Press about two-thirds of the crumb mixture into the bottom of the prepared baking pan to form the crust.
Spread the strawberry-rhubarb filling evenly over the crust.
Crumble the remaining crumb mixture over the 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 slicing into squares.