Bring a large pot of salted water to a boil. Boil potatoes, uncovered until tender. Drain potatoes and return them to the pan and cover.
Meanwhile, melt butter in a separate pan and saute onion (optional) and garlic until tender. Add cream (or half and half), spinach souffle, salt, pepper and nutmeg to the onion mixture. Stir together and heat through - do not boil.
Mash potatoes with hand mixer. Add onion/spinach mixture to the potatoes while mashing. Taste for seasoning. Place potatoes in a serving bowl. Cover loosely with foil and keep warm in the oven until ready to use.