I've never heard of pressure cooked potatoe soup.. but I make mine:
1.peel and boil potatoes ( as if to make mashed potatoes)
while potatoes are cooking I like to saute some chopped onions ( green is best but use what you have) in butter til translucent.
2. drain potatoes
3. add a stick of butter ( I make a BIG pot full )
4. coursely mash potatoes
5. add sauted onions
6. add milk until a bit thinner than mashed potatoes
7. simmer until desired thickness ( add more milk if u want it thinner)
opt.... I like to add a little roasted/minced garlice or garlic powder too