Step 1: Preheat your oven to 375°F (190°C). Lightly grease an 8x8 inch (20x20 cm) baking dish.
Step 2: In a large bowl, combine the chopped rhubarb, 3/4 cup granulated sugar, 2 tablespoons cornstarch (or flour), 1 teaspoon vanilla extract, and a pinch of salt. Toss gently until the rhubarb pieces are evenly coated.
Step 3: Pour the rhubarb mixture into your prepared baking dish, spreading it out evenly.
Step 4: In a separate medium bowl, whisk together the 1 cup all-purpose flour, 1/2 cup rolled oats, 1/2 cup packed light brown sugar, 1/4 cup granulated sugar, 1/2 teaspoon ground cinnamon, and 1/4 teaspoon salt.
Step 5: Add the cold, cubed butter to the dry ingredients. Using your fingertips, a pastry blender, or two knives, cut the butter into the flour mixture until it resembles coarse crumbs with some pea-sized pieces remaining.
Step 6: Evenly sprinkle the crumble topping over the rhubarb filling in the baking dish. Do not press it down.
Step 7: Place the baking dish on a baking sheet (to catch any drips) and bake for 40-45 minutes, or until the topping is golden brown and the rhubarb filling is bubbly and tender when pierced with a knife.
Step 8: Remove the crumble from the oven and let it cool for at least 15-20 minutes before serving. This allows the filling to set properly. Serve warm with your choice of accompaniments.