Peel and thinly slice potatoes using a mandolin. Evenly distribute potato slices among four greased ramekin.
In a microwave safe bowl, add butter and microwave for about 1 minute or until butter turns to liquid. Whisk in flour. Microwave for 1 minute more. Note: Cover with paper towel to prevent splattering.
Whisk heavy cream, milk and salt into the butter and flour mixture to form a roux-based creamy sauce.
Pour sauce evenly over potatoes in each ramekin. Be sure to completely cover potatoes with sauce then cover each ramekin with a piece of foil.
Add water to Instapot then insert trivet. Place two ramekins directly onto the trivet, then stack remaining ramekins on top becing careful to keep them all flat.
Cover Instapot, set pressure value to "Sealing" and manually set Pressure Cook time for 30 minutes. (You'll need to let pressure build for 10-15 minutes before cooking begins). When done and you hear the beep, release pressure and open when pressure pin has dropped. Note: If you potatoes are cut thick, they may require extra cook time.
Carefully remove each ramekin from Instapot (they will be hot) and top with cheese and any other toppings you desire then serve.