In a large bowl toss cauliflower and pears with oil and spread onto 2 large roasting sheets. Add 6 chestnuts to each pan. Roast until cauliflower is golden brown and chestnuts are soft, about 40 minutes.
As soon as chestnuts are cook enough to handle, peal them. Add cauliflower, nuts, pears, chicken broth to a large stock pot and puree with an immersion blender. Heat soup until warm. Taste and adjust seasonings.