In a saucepan, saute the sliced leeks in butter for approximately 3-5 minutes, until softened. Add the cabbage, and continue to saute until the cabbage is tender. Add chicken stock, potatoes and and carrots, along with about 1 tsp parsley and 1/2-1 tsp thyme. Bring to a gentle boil, then reduce heat and simmer for 20 minutes, or until all vegetables are soft.
Using an immersion blender, partially puree the soup for a creamy texture, although I prefer to leave some larger pieces as well. Salt and pepper to taste. (For a more formal presentation, puree until completely smooth, swirl in creme fraiche, and add a topping of crispy frizzled leeks).