• 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