You'll need fresh rhubarb. You can find it in the produce section, it looks like green and pink large celery stalks.
Clean and cut your rhubarb into one and a half inch pieces.
Cut the Rhubarb
Add the rhubarb and brown sugar to a medium size pot and cook over low heat.
Combine corn starch & lemon juice, whisk to combine until it is a smooth slurry. Once the brown sugar is fully melted add the slurry.
Make a Slurry
Cook over medium heat for roughly 5 minutes, mixing the whole time. Then transfer to your oven-safe dish.
Mix together and all-purpose flour, rolled oats, brown sugar, roasted walnuts, and salt.
Make the Crumble
Next, add the chunks of butter and use your hand to rub the butter into the dry mixture.
Add the Butter
The crumble is ready when you have a thick coarse meal texture.
Make a Glaze
Sprinkle over rhubarb and bake for 30 minutes, or until the top is golden brown and the rhubarb is bubbling up the sides.