Place potato slices (be sure to keep them as uniform and close to 1/4" as possible!) broth, garlic and onion in the pot. Close and lock lid, turning steam release valve to closed. Press Steam and adjust time to 3 minutes.
When cooking time is done, do a quick release and gently scoop out the potatoes into an oven safe dish and set aside. Leave any remaining liquid in the pot.
Press Start/Stop to turn off the Keep Warm setting and then press Brown/Saute, adjust to low and press Start/Stop again.
Add heavy cream, butter and parsley to the pot, stirring well to combine. Allow to simmer for 1 minute. If you want to thicken the sauce more, add a flour or cornstarch slurry at this time and simmer for a couple minutes more.
Press Start/Stop to turn off the pot and then remove the inner pot from the heat completely. Slowly stir in the cheese. Don't add it all at once-it will "break" and you'll get grainy cheese sauce.
Pour the finished cheese sauce over the potatoes in the pan. Top with additional cheese if desired.
Broil for 2-3 minutes on High to crisp up the top and brown the cheese.
Allow to sit and cool for a few minutes before serving. Garnish with more fresh parsley if desired.
Thicken sauce before adding the cheese with a flour slurry made with the cooking liquid if desired.The thickness of the potatoes is important for cook time and evenly cooked slices. If you sliced them a bit too thinly, change cook time to 2 minutes instead of 3.For making ahead: store in the refrigerator until ready to serve. Then heat in the oven at 350 until warmed through and finish off under the broiler for the crispy brown top.