Preheat your oven to 350°F (175°C). In a mixing bowl, cream together the butter, granulated sugar, and salt until light and fluffy. Gradually mix in the flour until the mixture resembles coarse crumbs.

Press the shortbread mixture evenly into the bottom of a greased baking pan. Bake in the preheated oven for about 15-20 minutes, or until lightly golden.

Prepare the filling by combining the rhubarb, granulated sugar, brown sugar, flour, and cinnamon in a large bowl. Toss until well-coated.

Once the crust is baked, spread the rhubarb filling evenly over the warm crust.

In a separate bowl, combine brown sugar, oats, and flour for the streusel topping. Mix well and sprinkle over the rhubarb filling.

Return the pan to the oven and bake for an additional 25–30 minutes, or until the streusel is golden brown.

After baking, cool completely on a wire rack for at least an hour before cutting into squares.