Preheat your oven to 375°F (190°C).

Line an 8x8-inch baking pan with parchment paper.

In a large mixing bowl, combine diced rhubarb, strawberries, tapioca starch, lemon juice, cinnamon, vanilla extract, salt, and brown sugar. Stir well and let sit.

Cream softened butter, brown sugar, and white sugar together until light and fluffy.

Gradually add in flour, oats, and a pinch of salt to the creamed mixture. Mix until just combined.

Press two-thirds of the dough into the bottom of the prepared pan to form the crust.

Transfer the fruit filling onto the crust, discarding any excess liquid.

Mix remaining dough with optional nuts and sprinkle on top of the fruit filling.

Bake for 30-35 minutes until topping is golden brown.

Let cool in pan for 2-3 hours before slicing.