Place a small pot of water on the stove on high to get heating up. Peel sweet potatoes and cut into 1" slices, then halve the slices. Add all to hot water and heat to boiling. When to a boil, turn down heat to a low boil and boil for 10 minutes or til fork tender. Drain in colander placed in the sink. Transfer potatoes to a food processor. Add rest of ingredients. Processor bowl will be very full but don't worry. Process til smooth and creamy. Transfer to 1 1/2 quart caserole dish, cover with lid and keep warm in low oven. Or refrigerate, remove 30 minutes before baking, and reheat in oven at 350 for 25-30 minutes or til heated through. (Covered.)Garnish with more orange zest if desired.
*I just use my fine grate hand grater, but you can use the fine side of any cheese grater for the zest. Keep rotating the orange so you only grate off the orange part and not any white part.