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

In a large mixing bowl, combine 2 cups of rolled oats, 1 cup of all-purpose flour, 1 cup of light brown sugar, 1/4 cup of granulated sugar, 1 teaspoon of baking soda, 1 teaspoon of cinnamon, and 1/4 teaspoon of salt. Stir the dry ingredients together until well mixed, then blend in 1/2 cup of softened unsalted butter using a pastry cutter or your fingers until the mixture resembles coarse crumbs.

Preheat your oven to 350°F (175°C) and grease a 9x13 inch baking pan. Press half of the oat mixture evenly into the bottom of the prepared pan to form a crust. Then, spread the cooled rhubarb filling over the oat crust, ensuring even coverage. Finally, sprinkle the remaining oat mixture on top of the rhubarb layer, distributing it evenly for a nice topping.

Place the assembled Amish Oatmeal Rhubarb Bars in the preheated oven and bake for 30-35 minutes, or until the top is golden brown and the edges are slightly crisp. Keep an eye on the bars; they should bubble slightly at the edges. Once baked, remove them from the oven and let them cool completely in the pan to firm up for easier slicing.