1. Preheat oven to 350°F. Lightly spray 13 x 9-inch pan with cooking oil.
2. Heat strawberries, rhubarb and lemon juice in sauce pan over medium heat for 5 minutes.
3. Add sugar and cornstarch; bring to a boil, stirring until mixture thickens. Remove from heat and cool.
4. Prepare cake batter according to package directions.
5. Pour half of cake batter into pan. Spread with strawberry mixture. Spoon remaining batter carefully over filling and bake 40 to 45 minutes. Serve garnished with whipped topping and strawberries.