An easy way to use your fresh rhubarb and strawberries with a box cake mix. Uses a 9 x 13 pan so you'll need a large serving plate to turn it upside down on. Or, you can divide it in two smaller round cake pans.
Preheat oven to 350F (325F for dark non-stick pans). Melt butter in rectangular pan, 9 x 13 inches in oven. Sprinkle brown sugar evenly over butter.
Arrange strawberries and rhubarb on brown sugar evenly. Press them into the brown sugar gently.
In a mixing bowl, beat cake mix with water, oil and eggs until smooth. Pour batter over strawberries and rhubarb.
Bake for 45 to 55 minutes or until toothpick is inserted in center and comes out clean.
Remove from oven and immediately run a knife around the sides of the pan to loosen the cake. CAREFYLLY, turn pan upside down onto a heatproof serving plate. Leave pan over cake for 1 minute so the brown sugar topping can drizzle over the cake.
Cool for 30 minutes. Serve warm or cool with a dolup of whipped cream on top. Store loosely covered.