Coarsely chop carrots and celery. Place mashed potatoes in a medium saucepan. Gradually add broth and milk, whisking until mixture is smooth. Stir in carrot, celery, garlic, salt and pepper. Bring to a boil; reduce heat. Simmer uncovered, 10 minutes.
Remove from heat; stir in sour cream and parsley. Ladle soup into bowls; top with desired toppings.