Preheat oven to 350°F (175°C). Lightly grease a 9×9-inch baking pan or line with parchment paper with an overhang.

In a large bowl, combine 1 ½ cups flour, 1 cup rolled oats, ⅓ cup brown sugar, 2 tablespoons granulated sugar, 1 teaspoon cinnamon, and ¼ teaspoon salt. Mix evenly.

Add cold, cubed ¾ cup butter. Use a pastry cutter or fork to cut butter into dry ingredients until crumbly with pea-sized bits.

Reserve 1 ½ cups (about 200g) of the oat mixture for topping. Press the remaining mixture firmly into the bottom of the prepared pan to form the crust.

In a medium bowl, toss 4 cups chopped rhubarb with 1 cup granulated sugar, 2 tablespoons flour, and 1 teaspoon vanilla extract until coated and sugar starts to dissolve.

Spread the rhubarb mixture evenly over the crust.

Sprinkle the reserved oat mixture evenly over the rhubarb layer.

Bake for 40-45 minutes until topping is golden brown and filling is bubbly.

Cool completely in the pan on a wire rack for at least 2 hours to set the filling.

Lift bars out using parchment overhang and cut into squares. Serve plain or with vanilla ice cream or whipped cream.