Preheat the Oven: Begin by preheating your oven to 350°F (175°C).
Prepare the Apples: Peel and core the apples, then slice them into thin wedges. Place them in a large mixing bowl.
Add Cinnamon and Sugar: Sprinkle the granulated sugar and cinnamon over the apples. Toss until the apples are evenly coated. Pour the apple mixture into a greased baking dish.
Make the Crumble Topping: In a separate bowl, combine the flour, rolled oats, melted butter, salt, and brown sugar (if using). Stir until the ingredients are well mixed and crumbly.
Assemble the Dessert: Evenly sprinkle the crumble topping over the apple mixture.
Bake: Place the baking dish in the oven and bake for 25-30 minutes, or until the apples are bubbly and the topping is golden brown.
Cool and Serve: Allow the dessert to cool for a few minutes before serving. It is delicious warm and can be paired with ice cream or whipped cream for an extra treat.