In a large saucepan, heat the olive oil. Add the parsnips, sunchokes and garlic and cook over moderate heat, stirring, until slightly golden, about 5 minutes. Add the garam masala, cumin, turmeric and salt. Cook, stirring, until fragrant, about 2 minutes. Add the water, vegetable broth and coconut milk. Bring to a simmer and cook until the vegetables are soft, about 20 minutes.
Purée the soup in a blender until very smooth. Pour the soup into a clean saucepan and stir in the lemon juice. Season with salt and pepper.
To make the topping, heat olive oil under low heat, add the walnuts and cook and stir until golden, about 3 minutes. Remove from the heat and stir in the garlic and lemon juice. Transfer mixture to a small bowl. Cool to room temperature, then stir in the pear, parsley and tarragon. Season with salt and pepper. Ladle soup into bowls and sprinkle with the topping.