Preheat your oven to 350°F (175°C).

In a large mixing bowl, combine rolled oats, flour, sugar, and cold butter. Mix until it resembles coarse crumbs. Reserve half for topping.

In a separate bowl, mix chopped rhubarb, sugar, and vanilla extract. Fold in lemon zest if using. Allow it to sit for 5 minutes.

Press reserved streusel mixture into the bottom of a lined baking pan. Bake for about 10 minutes.

Spread rhubarb mixture evenly over the crust.

Sprinkle remaining streusel mixture on top and press lightly to adhere.

Bake for 30 to 45 minutes until the topping is golden and filling is bubbling.

Cool completely in the pan before slicing into squares.