1-Prepare oven and pan: Preheat oven to 400°F (200°C). Grease a 9-inch square baking pan or line with parchment paper, leaving overhang.

2-Make crust dough: Mix 1 cup flour, 1 cup oats, and a pinch of salt. Rub in 1/2 cup softened butter until coarse crumbs, then add an egg and stir to form a soft dough.

3-Form and bake base: Press two-thirds of the dough evenly into the prepared pan; bake for 15 minutes until edges are lightly golden. Let cool slightly.

4-Cook rhubarb filling: In a saucepan, combine 4 cups chopped rhubarb, 1 cup sugar, lemon zest, and lemon juice. Cook over medium heat for 5-7 minutes, stirring occasionally, until rhubarb softens and bubbles.

5-Thicken filling: Stir in 2 tablespoons cornstarch. Continue cooking for 5 minutes, stirring constantly, until the filling thickens and becomes glossy. Remove from heat.

6-Pour filling: Pour the warm rhubarb filling over the pre-baked crust and spread it evenly.

7-Make crumble topping: Take the remaining dough, add a few tablespoons of cold butter (cut into small cubes), and rub in until coarse crumbs form. Optionally mix in 1/4 cup chopped nuts or 1/2 teaspoon cinnamon.

8-Top and bake: Sprinkle the crumble topping evenly over the filling. Bake for 20-25 minutes until the topping is golden brown and the filling is bubbling around edges.

9-Cool and slice: Cool the bars completely in the pan on a wire rack for 1-2 hours. Then lift out using parchment and cut into 9 squares.