Step 1: Preheat your oven to 375°F (190°C). Lightly grease a 9-inch square baking dish or similar-sized oval dish.
Step 2: In a large bowl, combine the chopped rhubarb, 3/4 cup granulated sugar, cornstarch, finely grated fresh ginger, lemon juice, and a pinch of salt. Toss gently until the rhubarb is evenly coated.
Step 3: Pour the rhubarb mixture into your prepared baking dish, spreading it out evenly. Set aside.
Step 4: In a separate medium bowl, whisk together the all-purpose flour, light brown sugar, 1/4 cup granulated sugar, ground cinnamon, and 1/4 teaspoon salt.
Step 5: Add the cold, cubed unsalted butter to the flour mixture. Using your fingertips, a pastry blender, or two knives, cut the butter into the dry ingredients until the mixture resembles coarse crumbs with some pea-sized pieces of butter remaining. Do not overwork the mixture.
Step 6: Sprinkle the crumble topping evenly over the rhubarb filling in the baking dish. Do not press it down.
Step 7: Place the baking dish on a baking sheet and bake for 35-45 minutes, or until the topping is golden brown and the rhubarb filling is bubbling vigorously around the edges. If the topping browns too quickly, tent loosely with foil.
Step 8: Once baked, remove the crumble from the oven and let it cool for at least 15-20 minutes before serving to allow the filling to set. Serve warm with ice cream or whipped cream.