Sauté celery and onion in hot oil in a large Dutch oven over medium-high heat 10 to 15 minutes or until tender. Add ginger, and cook 5 minutes.
Add carrots, parsnips, and chicken broth to Dutch oven; bring to a boil. Reduce heat, and simmer, stirring occasionally, 45 minutes or until carrots and parsnips are tender. Remove from heat; let cool slightly (about 5 to 10 minutes).
Process mixture, in batches, in a food processor or blender until smooth, stopping to scrape down sides.
Return purée to Dutch oven. Stir in half-and-half, salt, pepper, and, if desired, allspice and nutmeg; simmer, stirring occasionally, over low heat 10 minutes or until thoroughly heated.
