Peel the parsnips and sweet potato and cut into 1-inch pieces. Add the parsnips, sweet potato, onion, and ginger to a large pot. Add enough cold water to cover the vegetables. Squeeze the lemon juice into the water to keep the parsnips from turning dark. Bring the water to a boil and simmer for 25 minutes or until the vegetables are fork tender. Drain the vegetables, reserving the liquid. Add half of mixture with 1 cup heavy cream in a food processor. Pulse until smooth then add the remaining vegetables, salt, pepper, cayenne, cinnamon, and nutmeg. Pulse again until smooth. To thin the soup, add hot water from the boiling pot. Add a little at a time, pulsing after each addition. You can always add more liquid, but you can't take it out. When the desired consistency is achieved, transfer the soup to a serving bowl. If you need to reheat the soup you can warm it over medium heat. To each serving add a dollop of yogurt, and a sprinkling of candies pecans and chives. Serve warm or at room temperature.