Melt butter in a soup kettle/pot.
Add the onion and cook over low heat until wilted, about 10 minutes.
Add the carrots, celery, stock, 1/4 cup dill, salt, black pepper and cayenne. Bring to a boil, reduce heat and cover. Simmer until carrots are tender, about 40 minutes.
Allow to cool slightly. Puree the soup in batches in a blender.
Return to the kettle, stir in remaining 2 Tablespoons dill and adjust seasonings. Heat through.
Serve each bowl of soup garnished with sour cream and a sprig of dill or cilantro. (Be creative: draw a carrot, heart, or person’s name on top of soup with the sour cream or top with pepitas for a crunch before serving).