Strawberry Upside-Down Cake

2 C. chopped, fresh strawberries

6 oz. package dry strawberry Jell-O

3 C. miniature marshmallows

1 box white or yellow cake mix, batter prepared according to directions on package

8 oz. Cool Whip, optional

Preheat oven to 350 degrees F. Lightly spray 13" X 9" baking dish. Spread strawberries in bottom of pan. Evenly sprinkle dry Jell-O over strawberries. Top with miniature marshmallows. Pour cake batter over marshmallows. Bake 40-50 min. or until a toothpick inserted in the center comes out clean. Cool in pan 15 min. Run a knife around edges of pan & turn out onto cake plate. Top each serving with Cool Whip, if desired. Store in refrigerator.