Peel parsnips and potatoes and cut into medium size cubes. Heat olive oil in a large soup pot. Add shallots, sage and garlic and sauté until they begin to brown. Add potatoes and parsnips and stir everything together. Pour enough vegetable broth or water to cover vegetables. Bring to a simmer and cook until potatoes and parsnips are very tender. Using an immersion blender, blend everything until smooth. Add more water if consistency is too thick. Stir in cream and season with thyme and salt and pepper, to taste.