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Preheat oven to 400°F.
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Spread mushrooms, onion, fennel, garlic and thyme on large sheet tray or nonstick roasting dish. Sprinkle pieces of butter evenly over vegetables; drizzle liberally with olive oil. Season with salt and pepper; roast 25-30 minutes.
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Mushrooms and fennel should be tender and slightly caramelized when done. Remove tray from oven and transfer all ingredients (including pan juices) to large pot.
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Add chicken stock and bring to a boil; reduce heat and simmer uncovered for 15 minutes. Remove from heat and puree until completely smooth using handheld or regular blender.
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Stir in heavy cream; simmer gently for additional 5 minutes. Taste; adjust salt and pepper.
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While soup simmers, in small mixing bowl, combine dressing mix, sour cream and chopped chives.
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To serve, ladle soup into soup bowls and garnish each serving with ranch sour cream and chives.