Make the caramel apple topping (This goes on the bottom of the cake pan, but will become the top when inverted.)
Peel and core 2-3 apples. Cut 1-2 apples into thin slices (about 2-3 cm thick) and dice the other apple into small cubes. (You will need enough slices to cover the bottom of a cake pan and 1/2 cup of diced apples for the cake batter.)
Melt 4 tbsp of vegan butter, 1/2 cup brown sugar, 3 tbsp maple syrup, and salt in a small saucepan and simmer until it begins to boil and the sugar is dissolved.
Pour the sugar mixture into the bottom of a 9 X 2-inch round cake pan and then arrange the sliced apples in a pattern in the sugar mixture.
Make the cake
Preheat the oven to 350° F (177° C).
Measure out 1 1/2 cup flour, 3/4 cup sugar, 1 tsp baking soda, 1/2 tsp cinnamon, and 1/4 tsp salt in a mixing bowl and give it a stir.
Mix 1 cup plant-milk, 1 1/2 tsp apple cider vinegar, and 1/3 cup oil in a separate container and let it sit for a few minutes until the milk begins to curdle
Stir the wet ingredients into the dry ingredients until they are well incorporated.
Toss in 1/2 cup of diced apples and give another small stir.
Spoon the cake batter over top of the caramel and apple layer so that you don't disturb your pattern.
Bake the cake at 350° F (177° C) for about 35 minutes or until a toothpick inserted in the middle comes out clean.
Allow the cake to cool for 10 minutes, slide a knife around the edges if they appear to be stuck, then invert the cake onto a plate. To do this, set a plate (or cake stand) on top of the cake, then quickly flip the cake over so that the cake pan is now sitting on the plate. Tap the cake pan and wiggle it slowly until the cake releases and you can pull off the pan.
Notes
I use a heavy non-stick cake pan and the cake always comes out clean without having to grease the pan, but if you have a thin aluminum cake pan, you may want to cut a circle of parchment paper to line the bottom of the pan.
Allow the cake to cool for about 10 minutes to let the cake solidify enough that it will hold together when inverted, but don't let it cool too much or the caramel will start to harden and stick to the pan.
Heat the caramel just long enough for the brown sugar to melt and become smooth. Don't let it boil or it will start to crystalize.
If you don't want to make the cake from scratch, you can use any vegan boxed cake mix and follow the directions for my super easy vegan soda cake and spoon that on top of the caramel apple mixture.
It is common for the butter and sugar mixture to bubble up and over in upsidedown cakes. You may want to place an old cookie sheet or some foil in your oven under the cake pan to catch any drips.