Preheat your oven to 400°F (200°C) and grease a 9-inch square pan.

In a bowl, combine flour, oats, and salt. Mix in softened butter until it resembles coarse crumbs.

Add the egg to the mixture, forming a smooth dough. Press into the pan and bake for 15 minutes.

In a saucepan, combine rhubarb, sugar, lemon zest, and juice over medium heat until bubbling.

Sprinkle in cornstarch, stirring to thicken the mixture which should take an additional 5 minutes.

Pour the rhubarb filling over the crust, spreading evenly.

Combine remaining flour and oats, rub in cold butter until crumbly. Optionally add nuts or spices.

Sprinkle the crumble topping over the filling and bake for 20-25 minutes until golden brown.

Allow the bars to cool completely in the pan for 1-2 hours before cutting into squares.