• Adjust oven rack to center of oven and preheat oven to 350°F.

Rhubarb Filling

• Add chopped rhubarb to a 9" x 13" baking dish. Sprinkle lemon juice evenly over the rhubarb, then fold a few times to incorporate. Spread rhubarb into an even layer.
• Sprinkle sugar evenly over rhubarb, then do the same with the cornstarch. Fold a few times to evenly incorporate. Spread rhubarb into an even layer. With a paper towel, wipe interior sides of baking dish for a cleaner finished appearance.

Cake Layer

• Add dry cake mix to a medium mixing bowl. Drizzle melted butter over the top, then stir to evenly combine.
• Using your fingers, crumble cake batter mixture into ¼" to ½" crumbles. Sprinkle crumbles evenly over top of rhubarb, leaving about 1" of exposed rhubarb filling around the perimeter of the baking dish. Do not press on or smooth out the cake batter crumbles.
• Sprinkle coarse sugar evenly over the cake batter crumbles.
• Bake 45-50 minutes, or until rhubarb is lightly thickened, glossy, and bubbling, and the cake batter topping is lightly browned.
• This easy dessert is best enjoyed on the day it is made, although it keeps perfectly well if you want to make it 6-8 hours in advance >> I recommend that you not cover the pan; just let it sit at room temperature uncovered. Our family especially likes to eat this while it's still a bit warm, about 20-30 minutes after it comes out of the oven - and with scoops of vanilla ice cream, of course!