Preheat your oven to 350°F (175°C) and line a 9x9-inch baking pan with parchment paper.

In a large mixing bowl, combine all-purpose flour, old-fashioned oats, brown sugar, granulated sugar, salt, and baking powder. Whisk until well blended.

Cut in the cold, cubed butter to the flour mixture until the texture resembles coarse crumbs. Reserve about 1 cup of this mixture for topping.

Press the remaining crumb mixture into the bottom of the prepared pan to create a solid base layer.

In the same bowl, combine sliced strawberries, rhubarb, lemon juice, cornstarch, and additional sugar. Stir gently until the fruit is evenly coated.

Pour the fruit filling over the pressed crumb base and spread it evenly.

Sprinkle the reserved crumb mixture over the fruit filling.

Bake in the preheated oven for 50-60 minutes until golden brown and bubbly.

Remove from the oven and cool completely in the pan before lifting out with the parchment paper.

Once cooled, slice into squares and serve with whipped cream or vanilla ice cream if desired.