Preheat oven to 350℉. Grease or line a 9x13 baking sheet.
In a medium bowl, whisk together flour, cinnamon, baking soda, and salt.
Flax Egg: In a small bowl, whisk together ground flax with 6 tbsp water. Let it thicken for a few minutes.
In a large bowl, use a hand-held mixer to combine plant butter + brown sugar, until mixture becomes lighter and airy. Mix in the flax egg (or sparkling water half the amount at a time). Mix in vanilla.
Add in the dry mixture and the sour cream, alternating a little bit at a time. Fold the apples in gently.
Press mixture into the baking dish. Add the crumble topping evenly, and very gently press into cake. Bake 40 min. Check at 35.
Let it cool and set a few minutes. If desired, drizzle with caramel sauce. Serve warm.