Cut butternut squash in half and remove the seeds. Sprinkle both halves with a bit of salt and pepper and lay them face down in a shallow baking dish. Add about ¼ cup of water in the bottom of the pan, cover with foil and cook in a 375F oven for approximately 20-25 minutes, until tender. (This can be done ahead of time. You could even put the squash in the refrigerator to cool until the next day)
In a medium saucepan, cook onion over medium heat until fragrant, about 3-4 minutes. With a spoon, scoop the flesh out of the squash and add that to the pot, along with the pears, salt, pepper, cardamom, nutmeg, ground anise seeds and one cup of water. Bring to the boil then lower heat and simmer for about 5 minutes.
Remove from heat and allow to cool for a few minutes before transferring the soup to your food processor or blender; Process on high speed for a minute or two, until very smooth and creamy.
Meanwhile, mix yogurt, egg whites and the remaining cup of water together in a mixing bowl.
While the motor is running, slowly add that mixture to the soup that is already in your food processor or blender. You want to pour rather slowly and in a continuous stream so the eggs don’t cook and the yogurt doesn’t turn.
Transfer the soup back to the saucepan and bring this up in temperature, almost to a simmer. However, it is very important that you do now allow the soup to boil at this point, again, to avoid cooking the eggs and have that yogurt turn on you.
For a super smooth consistency (or if you have a few little specks of cooked eggs or yogurt that you want to get rid of), strain the soup through a fine mesh sieve before serving.