1. Pierce top of squash with fork, so the steam will escape, and bake about 45 minutes or until tender. Scoop out contents, deseed and set aside.
2. Sauté onions, celery, garlic, ginger, cumin, coriander: Heat olive oil on medium high heat in a large (4 to 6 quart) thick-bottomed pan. Add the onions and celery. Lower the heat to medium and cook until softened, 8 to 10 minutes.
3. Add the squash to the pot with the onions and celery mixture. Add the stock, salt and pepper. Increase heat to high to bring the soup to a simmer, then lower the heat to low, partially cover and cook 8 to 10 minutes.
4. Purée with immersion blender to desired thickness.
5. Top with pumpkin seeds.