Heat oil in a soup pot, add onion, carrots and garlic; saute until softened, about 5 minutes.
Add potatoes, bay leaf, and stock. Bring to a boil, reduce heat and simmer about 30 minutes.
Add Swiss chard and herbs and cook an additional 10 minutes.
Add black pepper. Remove from heat and puree with immersion blender (or in batches in a food processor or blender).
If serving hot, return to soup pot and reheat for 5 minutes. If serving cold, cover and refrigerate for several hours or overnight.
Serve with a dollop of sour cream.