In a large bowl, mix the diced apples with brown sugar, cornstarch, cinnamon, salt, and lemon juice until well coated.
Transfer the apple mixture to a greased 9x13-inch baking dish, spreading them out evenly.
In a separate bowl, combine the topping ingredients: flour, rolled oats, brown sugar, and cinnamon.
Pour the melted vegan butter into the flour and oats mixture, and mix until crumbly.
Sprinkle the crumbly topping evenly over the apple mixture in the baking dish.
Bake in a preheated oven at 350°F for about 45 minutes, or until the topping is golden brown and the apples are bubbly. Serve.