Cut the butternut in half cross-wise, separating the top cylindrical piece from the lower bulbous part. Cut the stem and bottom off. Peel each half separately with a vegetable peeler. Cut into half again. Remove seeds from bottom half. Cut squash flesh into large chunks. Put squash into a pot large enough to hold it with about 2 cups of water. Cover tightly and bring to boil. Steam 10-15 minutes until soft.
Meanwhile, melt butter in a large skillet and add onion and half the salt and the pepper.
Cook onion over medium-high heat for several minutes until soft. Add garlic and other spices. Reduce heat to medium and cook, stirring, until squash is tender.
In a food processor or heavy duty blender, add cooked squash, remaining 1/2 teaspoon salt, yogurt, Parmesan, and half the onions and butter.
Blend until smooth, adding up to 1/2 cup squash cooking liquid if necessary to make a puree.
Serve topped with remaining onions and butter. This can be kept hot in a warm oven if you need. Makes about 6 cups