Melt the butter in a large soup pot or dutch oven over medium heat. Add the vegetables, and toss to coat in the butter, cooking and stirring for a minute or two. Add the thyme, salt and water. Bring to a boil, then reduce to a simmer. Cook, partially covered, until the vegetables are very tender, 30 or 40 minutes. Serve with a splash of cream and a few fresh thyme leaves.