Preheat your oven to 350°F (175°C).
Peel, core, and slice the apples into thin wedges.
In a large bowl, toss the sliced apples with granulated sugar, lemon juice, and cinnamon.
Spread the apple mixture evenly in a greased baking dish.
In another bowl, combine oats, flour, brown sugar, and salt.
Pour melted butter into the dry mixture and stir until crumbly.
Evenly distribute the crumbly topping over the apple mixture.
Bake in the preheated oven for 45 minutes until the topping is golden brown.
Let cool for a few minutes before serving.
Enjoy warm, ideally with a scoop of vanilla ice cream!