Step 1: Preheat your oven to 375°F (190°C). In a large bowl, combine the chopped rhubarb, granulated sugar, all-purpose flour (or cornstarch), vanilla extract, and optional ground nutmeg. Toss gently until the rhubarb is evenly coated.
Step 2: Pour the rhubarb mixture into an ungreased 9×13 inch baking dish. Set aside.
Step 3: In a separate large bowl, whisk together the 1 1/2 cups all-purpose flour, rolled oats, light brown sugar, ground cinnamon, and salt.
Step 4: Add the cold, cubed butter to the dry ingredients. Using your fingertips, a pastry blender, or a food processor, cut the butter into the dry ingredients until the mixture resembles coarse crumbs with some pea-sized pieces of butter remaining.
Step 5: Evenly sprinkle the crumble topping over the rhubarb filling in the baking dish.
Step 6: Place the baking dish on a baking sheet to catch any potential drips. Bake for 45-55 minutes, or until the topping is golden brown and the rhubarb filling is bubbly and tender when poked with a knife.
Step 7: If the topping browns too quickly, you can loosely cover it with aluminum foil for the last 10-15 minutes of baking.
Step 8: Allow the rhubarb crumble to cool for at least 15-20 minutes before serving. This cooling time allows the filling to set properly. Serve warm with vanilla ice cream, whipped cream, or custard.