Carrot and Cilantro Soup

1 onion, sliced into thin rounds
3 Tbls butter
1 bunch carrots, sliced into thin rounds
1 quart chicken stock
½ bunch cilantro, coarsely chopped
¼ tsp cayenne pepper
Juice of 1 lime
Salt and pepper to taste

In a heavy-bottomed pot over medium-low heat, melt the butter. Add the onion and carrots and sauté until tender, 8-10 min. Add the chicken stock, bring to a boil, then lower the heat to a simmer. Simmer until vegetables are very soft, about 15 min. Add cilantro, cayenne, lime juice and seasoning. Puree, either with a hand-held immersion blender, or by allowing the soup to cool slightly before transferring to a blender or food processor. Serve with a dollop of sour cream in each bowl. Serves 2-4.