1 Tablespoon butter, plus 2 Tablespoons for optional garnish
In a large pot over medium heat, melt 1 tablespoon of the butter. Add the onions and salt. Cook, stirring occasionally and adjusting heat so onions are cooking but not browning, until onions look starchy and a bit creamy, about 5 minutes.
Add the garlic and cook until fragrant, about 1 minute.
Purée the soup with a hand-held blender. Or, whirl in batches in a blender or food processor until smooth (place a kitchen towel over blender to prevent potential burns).
Stir in the pepper and nutmeg. (Note: At this point the soup may be cooled, covered, and frozen for up to 4 months.)
Add the milk and cook over medium-low heat until the soup is hot. Taste and add more salt, pepper, and/or nutmeg to taste.
(Meanwhile, if you want to add the parsley butter swirl, melt the remaining 2 tablespoons of butter and stir in the parsley.)
Serve hot, with a swirl of parsley butter, if desired. - Serves 8 (1 cup each)