Melt 4 tablespoons butter in Dutch oven over medium heat. Stir in celery, carrots, onions. Cover the pot and reduce the heat to low. Let the vegetables cook for 30 minutes. Remove the lid. Add the potatoes and chicken stock and simmer until tender, about 30 minutes. Stir in cream and sea salt.
In a small saucepan melt remaining 4 Tbs. butter and arrowroot (whisking while incorporating to avoid clumping). Let the butter and flour mixture cook for a couple of minutes while constantly stirring. Stir the flour mixture into the vegetables and broth. Blend soup until creamy using a hand immersion blender or you can blend the soup in a blender in small batches. Adjust sea salt to taste.
Garnish with cheese and sour cream, green onions and pepper.