Preheat the oven to 180°C (fan 160°C).
Mix the rhubarb with the sugar, lemon juice, vanilla and cinnamon. If the rhubarb is especially juicy, stir in the corn starch. Transfer to a baking dish.
In a separate bowl, combine the flour, butter, sugar, chopped nuts, salt and cinnamon. Rub everything together with your fingertips until coarse crumbs form.
Sprinkle the crumble evenly over the rhubarb without pressing it down.
Bake for 30–35 minutes, until the topping is golden brown and the fruit is bubbling around the edges.
Let cool for about 10 minutes before serving. Enjoy warm on its own or with yogurt, kefir, skyr or a little vanilla yogurt.