Preheat oven to 350ºF. Grease an 8x8-inch baking dish with unsalted butter.
Core and slice the apples into 1/4-inch thick slices. In a large bowl, toss together the apples with the leftover cranberry sauce. Pour the fruit mixture into the prepared baking dish, spreading it into an even layer.
In a small bowl, whisk together the flour, sugar, brown sugar, cinnamon and oats until combined. Cube the butter then add it to the bowl, working it into the oat mixture with your fingers until it’s well incorporated. Sprinkle the oat mixture evenly over the fruit. Place the dish on a baking sheet and bake the crisp for 32 to 35 minutes, until the oats are toasted and the fruit is bubbling. Remove the crisp from the oven and allow it to cool for 5 minutes so the juices thicken slightly.
Serve the crisp warm topped with a scoop of ice cream or a dollop of whipped cream.