In a large saucepan, heat the oil over medium. Add the celery, onion, garlic, potatoes, and a pinch of salt. Saute for 8-10 minutes, stirring frequently.
Add the liquids, bring to a boil, then reduce the heat to medium-low. Cook for 10 minutes, until the potatoes are tender.
Remove from heat, add stir in the dill, parsley, and red pepper flakes (if using).
Transfer the soup to a blender and pulse until smooth and creamy. Season to taste.
Serve warm or chilled.