In a medium saucepan, combine 2 cups of chopped fresh rhubarb, 1/4 cup of water, 1 tablespoon of lemon juice, and 2 tablespoons of cornstarch. Cook over medium heat, stirring gently for about 5-7 minutes until the rhubarb softens and the mixture thickens slightly. Once ready, set this vibrant rhubarb filling aside to cool while you prepare the oat mixture.

In a large mixing bowl, combine 1 1/2 cups of rolled oats, 1 cup of all-purpose flour, 1 cup of light brown sugar, 1/2 cup of granulated sugar, 1 teaspoon of baking soda, 1 teaspoon of cinnamon, and 1/4 teaspoon of salt. Add 1/2 cup of softened unsalted butter to the dry ingredients, mixing with your hands or a fork until the mixture is crumbly.

Preheat your oven to 350°F (175°C) and grease a 9x13 inch baking pan. Press half of the prepared oat mixture evenly into the bottom of the pan. Spread the cooled rhubarb filling over the crust, then sprinkle the remaining oat mixture on top.

Carefully place the assembled bars into the preheated oven and bake for 30-35 minutes, or until the top is golden brown and the edges are bubbling slightly.

Once baked, remove the pan from the oven and allow the bars to cool completely in the pan on a wire rack. After cooling, cut the bars into squares or rectangles.