Pour water into Instant Pot. Wash potatoes and slice thin. Place in steamer basket then into the pot. Cover and seal. Pressure cook 1 minute. Vent and remove lid.
Remove basket. Dump the water and dry the pot. Turn Instant Pot to sauté. Add in butter and melt. Add onions and sauté until soft. Add in garlic and stir in flour.
Whisk in the half and half, salt, pepper and garlic powder. Cook on sauté until thickened. Turn off the Instant Pot. Stir parmesan into the sauce. Fold the potatoes in.