1Heat oven to 350°F (325°F for dark or nonstick pan). In 13x9-inch pan, melt butter in oven. Sprinkle brown sugar evenly over butter. Arrange strawberries on brown sugar; sprinkle evenly with rhubarb. Press strawberries and rhubarb gently into brown sugar.
2In large bowl, beat cake mix, water, oil and eggs with electric mixer on low speed 30 seconds, then on medium speed 2 minutes, scraping bowl occasionally. Pour batter over strawberries and rhubarb.
3Bake 46 to 52 minutes or until toothpick inserted in center comes out clean. Immediately run knife around sides of pan to loosen cake. Place heatproof serving plate upside down on pan; turn plate and pan over. Leave pan over cake 1 minute so brown sugar topping can drizzle over cake. Cool 30 minutes. Serve warm or cool with whipped cream. Store covered in refrigerator.