Melt the butter over medium heat in a large soup pot.
Add the carrots, onions, leeks, and celery, and cook until tender but not browned, about 10 minutes.
Stir in the thyme and mushrooms and cook for 5 minutes, until the mushrooms have softened.
Add the chicken stock, salt and pepper, and simmer, covered, for 30 minutes.
Puree the soup in a food processor or blender in as many batches as necessary.
Adjust seasoning to taste.
Serve in warm bowls with a sprinkling of chives and a drizzle of olive oil, if desired.
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