Step 1: Preheat your oven to 375°F (190°C) and lightly grease an 8x8 inch baking dish or a 9-inch pie plate.
Step 2: Prepare the rhubarb filling. In a large bowl, combine the chopped rhubarb, granulated sugar, light brown sugar, flour, grated ginger (if using), vanilla extract, and a pinch of salt. Toss gently until the rhubarb is evenly coated. Pour the mixture into the prepared baking dish.
Step 3: Make the crumble topping. In another medium bowl, whisk together the flour, rolled oats, light brown sugar, granulated sugar, cinnamon, nutmeg, and salt.
Step 4: Add the cold, cubed butter to the dry ingredients for the topping. Using your fingertips, a pastry blender, or a fork, cut the butter into the flour mixture until coarse crumbs form. A few larger pea-sized pieces of butter are desirable.
Step 5: Evenly sprinkle the crumble topping over the rhubarb mixture in the baking dish.
Step 6: Place the baking dish on a baking sheet (to catch any potential overflow) 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 fork.
Step 7: Remove from the oven and let it cool for at least 15-20 minutes before serving. This allows the filling to set slightly. Serve warm with a scoop of vanilla ice cream or a dollop of whipped cream.