Melt the butter in a large stockpot and cook the leeks, celery, carrot and turnip or about 10-15 minutes or until softened but not browned. Add the stock, cabbage, potatoes and squash or pumpkin and simmer for about 30-45 minutes or until everything is nice and tender, adding water if necessary. Adjust the flavor with salt and pepper, transfer to a tureen and add a knob of butter just before serving.