In a large mixing bowl, cream together 1 cup of softened unsalted butter, 1/2 cup of granulated sugar, and a pinch of salt until light and fluffy. Gradually add in 2 cups of all-purpose flour, mixing until just combined. Press the dough evenly into a greased baking pan.

Preheat your oven to 350°F (175°C). Bake the crust for about 10 to 12 minutes until lightly golden around the edges.

Prepare the filling by combining 4 cups of chopped fresh rhubarb, 1/4 cup of light brown sugar, 1 cup of granulated sugar, 1/4 cup of flour, and 1 teaspoon of ground cinnamon. Toss gently to combine.

Once the crust is ready, pour the rhubarb filling over the pre-baked crust, spreading it evenly.

In a separate bowl, mix together 1/2 cup of all-purpose flour, 1/2 cup of rolled oats, and 1/4 cup of brown sugar until crumbly. Sprinkle this mixture over the rhubarb filling.

Bake for an additional 30 to 35 minutes until golden brown on top and the filling bubbles slightly.

Allow the bars to cool completely on a wire rack before slicing into squares.