Preheat your oven to 170°C (338°F). Grease the sides and base of an 8x8" baking dish with vegan butter or oil.
Make the apple filling: Wash, peel, and remove the cores from the apples. Slice them into approximately 1cm cubes.
Add the sugar, lemon juice, vanilla extract, cornstarch if using it, and cinnamon to a large mixing bowl and whisk to combine. Add the chopped apples to the bowl and stir using a spatula. Make sure all the apples are completely coated in the juices.
Transfer the apples and juices to the bottom of the greased baking dish and set aside.
Make the crumble topping: To a medium bowl, add the flour, white and brown sugar, cinnamon, and sea salt, and whisk to combine. Add the butter cubes and use the tips of your fingers to rub the butter into the flour mix. You'll be left with a crumble consistency.
Distribute the crumble topping on top of the apple filling in an even layer.
Bake: Bake for 40-55 minutes, check the apples are cooked after the 40 minutes by piercing them with a skewer. If the apples are slightly al dente they will still have some shape, or you can cook them for longer for a softer filling. Tip: If the crumble is browning too quickly, simply cover the crumble with some foil and return to baking. Serving: Once cooked, remove the crumble from the oven and allow it to cool on a wire rack. You can serve it warm, after about 15 minutes of cooling. Or allow it to come to room temperature, cover, and refrigerate until ready to serve. Serve with some vegan whipped cream or vanilla ice cream.
Storage: Store in an airtight container or even better, covered in the dish in which you baked it. Refrigerate for up to 3 days.