Place celery, carrots, onion and potatoes in pressure cooker with chicken broth. Bring to pressure and cook with regulator slowly rocking for 4 minutes. Cool cooker and open. Allow soup to cool slightly. Remove half the vegetables and, using an immersion blender, puree the rest in the pot. Return vegetables to pot and add milk, bouillon, parsley, and pepper. Adjust seasonings to taste.
Simmer for 5 - 10 minutes or until heated through. Garnish individual servings with chives, cheese and bacon bits if desired.
Yield: 8 servings (2 quarts).