ADD THE CELERY SEEDS to a dry heavy-bottomed soup pot or Dutch oven set over medium heat. Toast the seeds, stirring frequently, until fragrant, about 1 minute. Add the olive oil and stir to coat the seeds and heat the oil. Add the chopped celery, onion, carrot, and garlic and season with the celery salt and a few cracks of pepper. Cook, stirring, until the onions are just translucent, about 5 minutes.
Add the potatoes and season with two good pinches of kosher salt (don't be shy) and a few twists of pepper, then give it all a good stir. Add the stock and bring the mixture to a slow boil. Reduce the heat to low and cook, stirring occasionally, until the potatoes are tender, about 30 minutes.
Remove the pot from the heat and carefully ladle about half of the soup into a blender. Add half of the butter and blend until the soup is completely smooth and creamy. Return the soup to the pot and repeat with the remaining soup and butter. Stir in the cream and taste for seasoning, adding more salt and/or pepper if you like.
Divide the soup among the bowls and garnish with the reserved celery leaves.
