Peel, core, and slice the apples into eighths or sixteenths. Place the apple slices in a large bowl and sprinkle the ground cinnamon over them. Gently toss to coat.

Pour the cinnamon-coated apples into an 8-inch square Pyrex dish or similar baking dish.

In a separate large bowl, combine all-purpose flour and brown sugar. Mix well, then add Quaker Oats and stir until combined.

Pour the melted butter over the mixture and mix again until the oats and flour absorb the butter, forming a crumbly topping.

Evenly sprinkle the crumbly topping over the apples in the baking dish.

Preheat your oven to 375°F (190°C) and bake the crisp for about 30 minutes, or until the topping is golden brown and the apples are tender.

Let the apple crisp cool for a few minutes. Serve warm or at room temperature, optionally with vanilla ice cream or whipped cream.