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