Into the pan, layer thinly sliced apples along the bottom, and up the sides a little. Don't worry about this being perfect, just make sure you're overlapping slices so you have the bottom and a portion of the sides covered in apples.
Sprinkle the apple pie spice on the apples.
Prepare the muffin mix according to the directions, combining it with the milk.
Pour the mix on top of the apples. Cover this bowl with foil or a silicone lid.
In the inner liner of your Instant Pot, place two cups of water, and a steamer rack. Place the foil-covered pot on the rack.
Cook the cake on high pressure for 20 minutes and let it NPR for 10 minutes.
Remove the carefully and let it cool for 5-10 minutes before overturning carefully.
To brown the top of the cake:
Turn on your oven broiler and sprinkle the overturned cake with sugar.
Broil until the sugar melts and caramelizes, about 5-7 minutes.
WHAT IS POT IN POT COOKING?
This is just a fancy way of saying you'll be placing some water in the inner liner of your Instant Pot.
Then, you're going to place a steamer rack in the liner, with the water below it.
You'll use a smaller pot that can fit inside your Instant pot to place the apples and the muffin mix into.
You're going to cover this cake pot with either silicone or foil to keep the condensation out and place it on top of the steamer rack.
Now you have liner +water + steamer rack +cake pan+lid.