In a large bowl, combine sugar and cornstarch. Add rhubarb and apples or strawberries; toss to coat. Spoon into an 8-in. cast-iron skillet or other ovenproof skillet. , In a small bowl, combine the oats, brown sugar, butter, flour and cinnamon until the mixture resembles coarse crumbs. Sprinkle over fruit. Bake at 350° until crisp is bubbly and fruit is tender, about 45 minutes. If desired, serve warm with ice cream.