Oatmeal-Rhubarb Bars

Preheat oven to 350°F. Combine all-purpose flour, uncooked oatmeal, and packed brown sugar in a mixing bowl.

Stir the melted unsalted butter into the flour-oatmeal mixture until evenly moistened and the mixture resembles coarse breadcrumbs.

Press half of the oatmeal mixture firmly into the bottom of a 9x9-inch baking pan to create an even base layer.

Combine sugar and cornstarch in a saucepan, stirring to break up any lumps. Add water and vanilla extract, mixing until smooth.

Place the saucepan over medium heat and stir constantly until the mixture comes to a boil, then cook for 1 minute until slightly thickened.

Add the rhubarb to the saucepan and stir gently to coat with the sugar mixture. Simmer for 8–10 minutes until the rhubarb softens but still holds its shape.

Pour the warm rhubarb filling evenly over the oatmeal base layer in the baking pan.

Distribute the remaining oatmeal mixture evenly over the rhubarb filling, pressing gently so it adheres but doesn't compress too much.

Bake in the preheated 350°F oven for 20–25 minutes until the top is golden brown.

Remove from the oven and let cool in the pan for at least 10 minutes before cutting into bars and serving.