In a large saucepan or casserole, heat up the butter over medium-low heat. Add the onion, leek, celery, garlic and jalapeno pepper chili. Cook, covered for 30 minutes, stirring occasionally.
Add the carrots, cabbage, potatoes, turnips, fennel, kohlrabi, celery root, broccoli, parsley, and enough stock to nearly cover.
Bring the contents to a boil; lower heat and simmer, uncovered, for 30 minutes or until very tender.
Puree the soup, working in batches as necessary, in a processor or blender. Rinse out the pan, pour the soup back into the saucepan, and bring the soup to a simmer.
Add cream and salt and pepper to taste; heat back up to simmer. Serve with garnish of sour cream and chives.
NOTE: At Thanksgiving dinner at The Four Seasons, this soup is served in a hollowed-out acorn squash. To serve it this way, slice off the tops and a thin sliver from the bottom of the acorn squash (this is so the squash can 'sit up' on its own) then hollow out the insides.
Pour in the finished soup and bake on a baking sheet, uncovered, at 190ºC until the squash is tender, about 30 minutes. The squash will be tender enough to be scraped from the shell and eaten with the soup.
The vegetables in this soup can be varied according to what is in season, as long as you maintain the proportion of solids to liquid.