Rhubarb Upsidedown Cake

4 c. rhubarb, diced
3 c. miniature marshmallows
1/2 c. sugar
1/2 box white cake mix
1 (3 oz.) package strawberry Jello

Preheat oven to 350 degrees. Place ingredients in a greased 13x9x2-inch baking dish in the following order:
rhubarb, sugar, dry jello, marshmallows. Top with dry cake mix and a little butter. Bake for 45 minutes.