In a sauce pan over medium heat, melt the butter. When it begins to foam, add the onions, zest and ginger, and saute until the onions are translucent, about 10 minutes. Add the lemon juice, salt and pepper; continue to saute for 2 to 3 minutes longer.
Add the water, raise heat to high and bring to a boil. Cover, reduce heat to medium, and simmer until the broth is golden and the onions have almost disintegrated, about 30 minutes.
Remove from heat and strain through a sieve into a clear container; discard the contents of the sieve. Working in batches, if necessary, combine the strained stock and the potatoes in a blender or food processor and puree until smooth (hand-held-emersion blender works nicely). Transfer the puree to a clean saucepan and bring to a gentle boil.
Ladle into warmed soup bowls and garnish each serving with a little of the parsley.