Add the flour, sugar, and salt to a mixing bowl. Mix to combine. Add the coconut oil, milk, and vanilla, then use your hands to mix until the oil is well incorporated and the mixture is crumbly. Set aside.
Core and dice the apples, then place them in a large baking dish (approximately 31x21cm (~9×13 in.) or equivalent square or round dish). Add the sweetener and vanilla, and mix to coat the apples. Mix in the starch, cinnamon, and nutmeg to coat.
1 kg (2 lb.) apples, 3 tablespoons maple syrup or honey, 1 teaspoon vanilla extract, 1 tablespoon corn starch, 1 teaspoon cinnamon, ¼ teaspoon nutmeg
Sprinkle the topping in an even layer over the apple filling. Bake for 45-50 minutes, until the edges of the crumble are bubbling and the topping is dry to the touch and a deeper golden colour.
Cool for at least ten minutes before serving. Keep leftover crumble in a sealed container in the refrigerator for up to five days.
Notes
Choose a good slightly tart baking apple like Belle de Boskoop. I don't recommend granny smith or something that's just sour with no flavour. For the very best flavour, use a heritage variety.Storage: cool and keep leftovers in a sealed container in the refrigerator for up to five days. I recommend reheating in the oven to re-crisp the topping rather than a microwave, which will make the crumble soggy.Freezing: transfer full cooled crumble to an airtight container and freeze for up to three months. Thaw in the refrigerator before reheating in the oven until heated through.Make Ahead: prepare the crumble and refrigerate, unbaked, for up to one day. Make sure it's tightly covered with something like beeswax wrap. Bake as instructed directly from the refrigerator and add an extra five minutes to the baking time. The unbaked crumble can also be frozen for up to one month and thawed in the refrigerator before baking. If making ahead, you can mix in about a tablespoon of lemon juice to the apple filling to prevent browning.Bake until bubbling: the starch needs to come to a boil in order to thicken the apple filling properly. If the crumble isn't bubbling around the edges, it's not fully cooked yet (it will definitely bubble by the 50 minute mark - if it hasn't at all, your oven is probably running very cold).Don't peel the apples: you won't notice the peel on a good eating apple - again, don't use granny smith, they are the worst - and you'll save time and prevent food waste too.Try add-ins: if you like a nutty crumble, add a handful of finely chopped walnuts or sliced almonds to the streusel topping. Raisins and dried cranberries are good mixed in with the apples but make sure they're covered in the crumble or they'll burn.While honey isn't strictly vegan, I choose to use it because it's the most environmentally friendly choice (local to me - so no shipping needed - and organic, packaged in reusable glass jars, no additional energy needed for processing). Honey, if from a good source, is overall much better for animals than something like agave if you live in the global north. If you're very strict you can use maple syrup instead.