Step 1: Preheat the Oven
1. Preheat Oven: Preheat your oven to 350°F (175°C).

Step 2: Prepare the Filling
1. Mix Filling Ingredients: In a large mixing bowl, combine the sliced apples, cranberries, granulated sugar, cinnamon, lemon juice, and cornstarch (if using). Toss until the fruit is evenly coated.
2. Transfer Filling: Pour the fruit mixture into a greased 9×13-inch baking dish or a similarly sized oven-safe dish.

Step 3: Prepare the Crisp Topping
1. Mix Topping Ingredients: In a separate bowl, combine the rolled oats, flour, brown sugar, granulated sugar, cinnamon, salt, and chopped nuts (if using).
2. Add Butter: Pour the melted butter over the oat mixture and stir until everything is well combined and crumbly.

Step 4: Assemble and Bake
1. Spread Topping: Evenly spread the crisp topping over the fruit filling in the baking dish.
2. Bake: Bake in the preheated oven for 40–45 minutes, or until the topping is golden brown and the fruit is bubbling.

Step 5: Serve
1. Cool Slightly: Allow the crisp to cool for about 10–15 minutes before serving.
2. Enjoy: Serve warm, optionally topped with vanilla ice cream or whipped cream for an extra treat!