Step 1: Preheat your oven to 375°F (190°C). Lightly grease a 9x13 inch baking dish.
Step 2: In a large bowl, combine the chopped rhubarb with 1/2 cup granulated sugar, 2 tablespoons all-purpose flour, and 1 teaspoon orange zest. Toss gently until the rhubarb pieces are evenly coated.
Step 3: Pour the rhubarb mixture into the prepared baking dish and spread it out evenly.
Step 4: In a separate medium bowl, whisk together 1 cup all-purpose flour, 1/2 cup packed light brown sugar, 1/2 cup granulated sugar, 1 teaspoon ground cinnamon, and 1/4 teaspoon salt.
Step 5: Add the cold, cubed butter to the dry ingredients. Using your fingertips or a pastry blender, work the butter into the dry ingredients until the mixture resembles coarse crumbs with some pea-sized pieces remaining.
Step 6: Evenly sprinkle the crumble topping over the rhubarb mixture in the baking dish.
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 bubbly and tender when pierced with a fork.
Step 8: Remove from the oven and let cool for at least 15-20 minutes before serving. Serve warm with ice cream or whipped cream.