Dice the onion, potatoes, carrots and pumpkin. Sweat the onions and carrots in the hot fat. Add the potatoes and pumpkin. Pour in the broth. Season with salt, pepper and bay leaf. Cook for about 20 minutes.
Cut the chives into rolls. Take out half of the vegetables. Puree the other half. Mix the buttermilk and cornstarch, add to the soup.
Put the removed vegetables back into the soup, bring the whole thing to the boil briefly. Season with salt and pepper and sprinkle with chives and a little dill.