Melt 2 tablespoons butter in a large soup pot over medium-low heat.
Add onions & garlic. Saute - stirring often - until softened, about 3-5 minutes.
Add coriander, salt, pepper, and cook 1 more minute, while stirring.
Add lettuce, potatoes, and chicken broth.
Bring to boil, then reduce to a simmer and cover. Cook until potatoes are very tender, about 10 minutes.
Puree your soup - in batches - using the method of your choice. We've used a blender, food processor, and emulsion blender - they all work. Please be careful when blending hot liquids.
Transfer the blended soup to a small saucepan, adding the last tablespoon of butter and salt & pepper to taste. Heat through before serving.
Top with parmesan cheese & a dollop of sour cream.
Store leftovers in the fridge for up to a week or portion and freeze for soup on-the-go!