Dice the potatoes and onions.
Melt butter in a pot.
Add the onions and sauté until translucent.
Add the potatoes and sauté for a little longer.
Pour in water or broth and boil until the potatoes are cooked through.
Blend the potatoes and onions until smooth.
Return the blended mixture to the pot, add milk and salt, and bring to a boil once more.
Serve in a bowl, garnished with parsley or croutons.