Preheat the oven to 180c (160c fan).
Line a 20cm square cake tin with baking parchment.
Chop your apples into 1cm dice and place in the bottom of your lined tin. Sprinkle over 1tbsp coconut sugar.
Add the vinegar or lemon juice to the plant milk and allow it to sit for 5 minutes. The milk will become slightly lumpy. This is fine!
Blitz the oats to a flour in a blender or food processor and transfer them to a mixing bowl.
Quickly blitz the walnuts so that they are very finely chopped (or chop them by hand), and transfer to the mixing bowl.
Add the remaining dry ingredients to the bowl.
Add the milk mixture and vanilla extract to the dry ingredients and mix gently.
Pour the batter carefully over the apples, making sure the apples are covered.
Bake in the centre of the oven for 35-40 minutes until a skewer inserted comes out clean.
Once baked, place a cooling rack over the top of the cake tin and turn the whole thing upside down to turn the cake out. Peel the baking parchment from the top of the cake to reveal the apple layer.
