Add ground beef and onion to the Instant Pot. Using the display panel select the SAUTE function.
Brown the meat until no pink remains. Drain and discard liquids.
Spread beef in the bottom of an IP-friendly casserole dish, sprinkle with salt, then layer spinach (if using) and shredded cheese.
In a medium bowl, whisk together baking mix, milk and eggs to form a thin batter. Pour evenly over the meat and cheese. Cover loosely with foil--do not seal.
Rinse out the pot, then pour in one cup of water and insert the steam rack. Carefully lower the casserole on to the steam rack.
Turn the pot off by selecting CANCEL, then secure the lid, making sure the vent is closed.
Using the display panel select the MANUAL or PRESSURE COOK function*. Use the +/- keys and program the Instant Pot for 20 minutes.
When the time is up, let the pressure naturally release for 5 minutes, then quick-release the remaining pressure.
(Optional) Sprinkle additional shredded cheddar and set under the broiler for 3 - 5 minutes until cheese is bubbly.
Allow the casserole to set up for 5 - 10 minutes for easier serving.
Notes
*The MANUAL and PRESSURE COOK buttons are interchangeable.