Coat a 2.5 quart baking dish with cooking spray. Preheat oven to 350 degrees.
Filling
In a large bowl, combine the rhubarb, strawberries, and sugar until well mixed. Pour the mixture in to the prepared baking dish and set aside.
Topping
Combine the flour, oats, sugars, cinnamon, and salt until well blended. Stir in the melted butter until moistened crumbs form. Spoon the crumb mixture evenly over the filling.
Bake at 350 degrees for 35-40 minutes. Let cool for 10 minutes and serve with ice cream (optional).