Instructions

1. 1

   Preheat the oven to 350°F and line a 9x13-inch baking pan with parchment paper. Leave a little overhang on the sides so the bars lift out easily later.

2. 2

   In a large bowl, stir together the oats, flour, brown sugar, granulated sugar, baking powder, cinnamon, and salt until evenly mixed. Pour in the melted butter and stir until the mixture looks like damp sand with small crumbles throughout.

3. 3

   Press about two-thirds of the oat mixture firmly into the bottom of the prepared pan. The layer should look compact and even with no dry patches. Bake for 12 minutes, just until the edges start turning lightly golden.

4. 4

   While the crust bakes, combine the rhubarb, sugar, cornstarch, lemon juice, and vanilla in another bowl. The rhubarb will start releasing liquid after a few minutes, which helps create the soft jam-like filling.

5. 5

   Spread the rhubarb mixture evenly over the warm crust, including any juices in the bowl. Sprinkle the remaining crumble mixture over the top in loose clumps so some filling still peeks through.

6. 6

   Bake for 40 to 45 minutes, until the top is golden brown and the filling bubbles around the edges. The center may still look slightly soft, but it firms up as the bars cool.

7. 7

   Cool completely before slicing. Many home cooks find that chilling the pan for 30 minutes after cooling makes cleaner cuts, especially for oatmeal rhubarb bars with a thicker filling layer.