Preheat your oven to 350°F (175°C). Lightly grease a 9×9-inch baking dish or a 10-inch cast-iron skillet.
In a large bowl, gently toss the sliced apples, cranberries, granulated sugar, flour, lemon juice, cinnamon, nutmeg, and salt.
In a separate bowl, combine the oats, flour, brown sugar, cinnamon, and salt. Using your fingertips or a pastry cutter, work the cold butter into the dry ingredients until the mixture resembles coarse crumbs with some pea-sized butter pieces remaining.
Transfer the fruit mixture to your prepared baking dish, ensuring an even distribution. Sprinkle the oat crumble topping uniformly over the fruit, covering it completely.
Place in the preheated oven and bake for 40-45 minutes, until the topping is golden brown and the fruit filling is bubbling around the edges.
Allow the crisp to cool for 15-20 minutes before serving. This cooling period allows the juices to thicken and the flavors to meld together.