In a large stockpot, add butter, olive oil, onions, and curry powder. Cook, uncovered, over low heat for 15 to 20 minutes, until onions are tender. Stir occasionally, scraping bottom of pot.
Add squash, applesauce, salt, pepper, and chicken stock to pot. Bring to boil, cover, and cook over low heat for 30 to 40 minutes, until squash is very soft. Use an immersion blender to blend mixture until smooth. Add apple cider and enough water to make soup the consistency you like; it should be slightly sweet and quite thick. Season with salt and pepper and serve hot with pumpkin seeds (optional).