Combine potatoes, zucchini, chives, and stock in soup kettle. Bring to a boil, reduce heat, and simmer, partially covered, 15 minutes or until vegetables are tender. Remove from heat, add lovage and thyme, and let sit 5 minutes. Press through food mill. Cool and chill. Stir in yogurt and cream and season with salt and pepper. Garnish with matchstick-size slivers of raw zucchini.
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