Preheat the oven to 190℃. Lightly grease a medium baking dish and set aside.
Make The Filling:
In a large bowl, toss the chopped rhubarb with the sugar, cornstarch, lemon zest, lemon juice, vanilla extract, and salt.
500 g rhubarb, 100 g light brown sugar, 2 T corn starch, ½ lemon, 1 t pure vanilla extract, pinch table salt
Spread evenly into your prepared baking dish.
Make The Topping:
In a medium bowl, combine the flour, rolled oats, brown sugar, chopped walnuts, cinnamon, and salt.
60 g all-purpose flour, 45 g rolled oats, 100 g dark brown sugar, 60 g chopped walnuts, ½ t ground cinnamon, ¼ t table salt
Add the cold, cubed butter. Use a pastry cutter or your clean fingers to rub the butter into the dry ingredients until the mixture resembles coarse, pea-sized crumbs.
60 g unsalted butter
Assemble and Bake:
Sprinkle the walnut streusel topping generously over the rhubarb mixture.
Bake for 35 to 45 minutes, or until the topping is golden brown and the fruit is bubbling actively at the edge