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 chopped apples for the cake batter.)
Melt 4 tablespoons of vegan butter, 1/2 cup of brown sugar, 3 tablespoons of maple syrup, and salt in a small saucepan. Simmer until it begins to boil and the sugar is dissolved.
Pour the sugar mixture into the bottom of a 9x2-inch round cake pan, and then arrange the sliced apples in a pattern on top of the sugar mixture.
Make the cake
Preheat the oven to 350°F (177 °C).
Measure out 1 1/2 cups of flour, 3/4 cup of sugar, 1 teaspoon of baking soda, 1/2 teaspoon of cinnamon, and 1/4 teaspoon of salt in a mixing bowl and stir them together.
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.
Add 1/2 cup of diced apples and give the mixture another brief stir.
Spoon the cake batter over the 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 center 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 needing to grease the pan. However, 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, allowing it to solidify enough to hold together when inverted. However, do not let it cool too much, as 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 crystallize.
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; then, spoon that on top of the caramel apple mixture.
It is common for the butter and sugar mixture to bubble up and over in upside-down cakes. You may want to place an old cookie sheet or some foil in your oven under the cake pan to catch any drips.