Mix together the spinach, artichokes, cream cheese, mozzarella cheese, Parmesan, mayonnaise, sour cream, onion powders, garlic, and nutmeg in a pressure-proof container.
Cover the container with foil, sealing tightly around the edge so that no condensation can seep in.
Add 1 1/2 cups of water to the Instant Pot insert, and place the trivet into the bottom.
Put the container on top of the trivet, then cover Instant Pot with lid, seal, and set to Pressure Cook or Manual on High for 10 minutes and cook until done.
Allow for a Natural Pressure Release of 5 minutes, then perform a quick pressure release to release the rest of the pressure.
Remove the container from the Instant Pot and carefully discard the foil.
Stir the cheesy dip and then serve.