Preheat your oven to 350°F (175°C).
In a large bowl, combine sliced apples, granulated sugar, cinnamon, and lemon juice.
Toss the apple mixture until well coated.
Transfer the apple mixture to a greased baking dish.
In a separate bowl, mix the oats, flour, brown sugar, and salt.
Pour melted butter into the oat mixture and stir until crumbly.
Spread the oat mixture evenly over the apples in the baking dish.
Bake in the preheated oven for 40-45 minutes or until the topping is golden brown.
Remove from the oven and let it cool for a few minutes.
Serve warm, optionally with vanilla ice cream.