Creamy Celery and Potato Soup

I- Ingredients for 8 Servings:

1 tbsp light butter;
1 tbsp olive oil;
2 large leeks (white and pale green parts only), thinly sliced (about 2 cups);
1 large onion, chopped;
2 medium-size white potatoes (about 12 ounces), cut into 1-inch cubes;
2 medium celery roots (celeriac; about 1 1/2 pounds total), peeled, cut into 1/2-inch cubes;
1 tsp thyme;
1/2 tsp salt;
1/2 tsp garlic powder;
1 bay leaf;
6 cups fat-free low-salt chicken broth;
6 medium celery ribs chopped;
1/2 cup fat-free half-and-half;
1 tsp salt;
1 tsp pepper;
1 1/2 tsp lemon juice;
1/4 tsp cayenne pepper (optional).

Melt butter with oil in pressure cooker pot using sauté/brown.
Add leeks and onion and sauté/brown until almost tender, about 5 minutes.
Stir in potatoes, celery roots, thyme, garlic powder, salt, and bay leaf. Add broth.
Set to high pressure and cook for 10 minutes. Use quick pressure release.
Open the lid and add celery stalks.
Set on high pressure for 4 minutes. Use quick pressure release when done.
Add salt, pepper, lemon juice, cayenne pepper (if using) and fat-free half-and-half.
Remove the bay leaf. Using an immersion blender, puree soup in a pot.
Ladle into bowls and serve hot.