In a saucepan, combine the rhubarb, sugar, water, and lemon juice. Bring to a simmer over medium heat. Cook for about 10–12 minutes, stirring occasionally, until the rhubarb breaks down and thickens. Set it aside to cool slightly.

Tip: Keep stirring gently so it doesn’t stick to the bottom of the pan. It should look like a loose jam.

In a bowl, stir together the flour, oats, sugar, and salt. Pour in the melted butter and mix until everything’s moistened. It’ll look crumbly, but should hold together when pressed.

Press about 2/3 of the oat mixture into a greased 8×8 pan to form the crust. Spread the rhubarb filling evenly over the crust. Sprinkle the remaining oat mixture on top.

Finish with a light dusting of cinnamon sugar.

Bake at 350°F (175°C) for 35–40 minutes, until the top is golden and the filling bubbles at the edges.

Let the bars cool completely in the pan before slicing. They’ll set up as they cool, so don’t rush this part.