Rinse squash, cut in half, trim, remove seeds and dice. Rinse carrot and celery, trim, peel if necessary and cut into very small cubes. Rinse leeks, trim and cut white and light green portion into thin rings. Peel onions, chop and sauté with leeks and butter in a pan. Add carrot, celery and half of pumpkin cubes and season with curry powder and black cumin.
Add broth and cook for 15 minutes over low heat. Add remaining diced pumpkin and simmer another 10 minutes. Cook and season with salt and pepper. Stir the pot well, so that pumpkin cubes dissolve and create a smooth soup.