Grease a 9-inch by 13-inch baking dish with the butter.
Add the strawberries, rhubarb, sugar and cornstarch to the baking dish and toss with a wooden spoon until well combined and the cornstarch and sugar are coating the fruit.
In a medium bowl, add the white and brown sugar, the oats, the flour, the cinnamon and the salt and stir well come combine.
Add the melted butter and stir until well combined.
Pour the topping evenly over the fruit mixture in the baking pan.
Cover with aluminum foil and bake for 30 minutes at 350 degrees Fahrenheit.
Remove the aluminum foil after 30 minutes and bake for an additional 15-20 minutes, or until the topping is golden brown and the fruit filling is bubbly.
Let sit at room temperature for about 5-10 minutes before serving with vanilla ice cream.
**I don't recommend using frozen strawberries for this recipe as frozen strawberries release too much liquid. Feel free to use frozen rhubarb, although I recommend thawing it before using and draining any excess liquid.