Heat the oil in a large saucepan over moderate heat and cook the onion for 5 minutes, until tender but not brown.
Add the remaining ingredients and bring to a boil.
Reduce the heat and simmer covered 15 to 20 minutes, until the potato and squash are tender.
Carefully process in small batches in a food processor or electric blender until smooth.
Garnish with an additional grating of fresh nutmeg, if desired.
Calories: 139kcal | Carbohydrates: 27g | Protein: 1g | Fat: 4g | Saturated Fat: 0.5g | Polyunsaturated Fat: 2g | Monounsaturated Fat: 1g | Sodium: 10mg | Potassium: 541mg | Fiber: 3g | Sugar: 15g