In a medium saucepan, combine 2 cups of chopped fresh rhubarb, 1/2 cup of water, 1 tablespoon of lemon juice, and 2 tablespoons of cornstarch. Cook over medium heat, stirring occasionally, until the rhubarb softens and the mixture thickens, about 5-7 minutes. Once cooked, remove from heat and allow the filling to cool while you prepare the oat mixture.

In a large mixing bowl, whisk together 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 1/2 teaspoon of salt. Add 1 cup of softened unsalted butter to the dry ingredients and use your hands or a fork to mix until the mixture resembles coarse crumbs with a few larger clumps still remaining.

Preheat your oven to 350°F (175°C). Grease a 9x13 inch baking pan. Press half of the oat mixture firmly into the bottom of the prepared pan to create an even layer. Carefully spread the cooled rhubarb filling over the oat base, ensuring it’s evenly distributed, then sprinkle the remaining oat mixture on top to cover the filling.

Place the assembled bars in the preheated oven and bake for 30-35 minutes until the top is golden brown and the edges are bubbling. Once they're done, remove the pan from the oven and set it on a wire rack to cool completely before cutting them into squares.