Preheat oven to 425°F. In a large pot cook macaroni according to package directions; drain. Transfer macaroni to a large bowl.
Meanwhile, in a medium saucepan cook sweet potato, covered, in enough boiling water to cover 8 to 10 minutes or until tender; drain.
For sauce, in a blender or food processor combine sweet potato and onion. Cover and blend or process until smooth, adding enough of the milk to reach a creamy consistency. Add yeast, garlic, nutmeg, and rosemary. Cover and blend or process just until combined. Season with salt and pepper.
Pour sauce over cooked macaroni; stir to coat. Transfer to a 2-qt. square baking dish. Bake 15 minutes or until top starts to brown