1. Preheat and prep
Preheat your oven to 350°F (175°C). Grease or line your baking pan with parchment paper.

2. Make the oat crust
In a large mixing bowl, combine oats, flour, brown sugar, baking soda, and salt. Pour in the melted butter and mix until crumbly.

3. Press and bake the base
Press about 2/3 of the oat mixture firmly into the bottom of the pan to form the crust. Bake for 12–15 minutes, until lightly golden.

4. Cook the rhubarb filling
In a saucepan over medium heat, combine chopped rhubarb, sugar, flour, and water. Cook until the mixture thickens and rhubarb softens, about 5–7 minutes. Stir frequently to prevent sticking.

5. Assemble the bars
Pour the rhubarb filling evenly over the baked crust. Sprinkle the remaining oat mixture on top.

6. Bake again
Return to the oven and bake for another 25–30 minutes, or until the top is golden and bubbling around the edges.

7. Cool and slice
Let the bars cool completely before slicing. This helps the filling set and makes for cleaner cuts