Strawberry rhurbard upside down cake

1/4 cup butter or margarine
1 cup packed brown sugar
2 cups sliced fresh strawberries & 2 cups chopped fresh rhubarb or strawberry rhubard jam by Tastefullysimple.com is much easier and yummy
1 box Betty Crocker® SuperMoist® French vanilla cake mix
1 1/4 cups water
1/3 cup vegetable oil
3 eggs
1.Heat 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.
2.In large bowl, beat cake mix, water, oil and eggs with electric mixer on low speed 30 seconds. Beat on medium speed 2 minutes, scraping bowl occasionally. Pour batter over strawberries and rhubarb.
3.Bake 43 to 50 minutes or until toothpick inserted in center comes out clean. Immediately run knife around side 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. Store loosely covered at room temperature.