Melt butter in a large Dutch oven over medium heat. Add onion and celery, and sauté 5 minutes or until vegetables are tender.
Add chopped sweet potato, milk, and chicken broth; bring to a boil, stirring occasionally. Reduce heat, and simmer, stirring occasionally, 18 to 20 minutes or until sweet potato is tender. Remove mixture from heat, and stir in half of the pimientos. Add Cheddar cheese and next 4 ingredients, stirring until cheese melts. Let cool 5 minutes.
Process cheese mixture, in batches, using a handheld blender, blender, or food processor until smooth, stopping to scrape down sides. Stir in remaining half of pimientos before serving.
*1 medium potato, peeled and chopped, may be substituted.
Note: Our Test Kitchens professionals tested this soup using three different tools--a food processor, blender, and handheld blender. Though each worked well, the handheld blender produced the smoothest soup.