• Step 1: Preheat your oven to 350°F (175°C). Grease a 9×13-inch baking dish with butter or non-stick cooking spray.
• Step 2: In the prepared baking dish, spread the chopped rhubarb in an even layer. Sprinkle the granulated sugar evenly over the rhubarb.
• Step 3: Sprinkle the dry yellow cake mix over the rhubarb and sugar, covering it completely. Drizzle the melted butter evenly over the top of the cake mix.
• Step 4: If desired, sprinkle the chopped walnuts over the top of the cake mix and butter.
• Step 5: Bake the rhubarb dump cake for 45-55 minutes, or until the top is golden brown and the rhubarb is bubbling around the edges.
• Step 6: Allow the cake to cool for at least 15 minutes before serving. Serve warm, with a scoop of vanilla ice cream or whipped cream, if desired.