In a large pot of rapidly boiling salted water cook fresh spinach (if using) for 1 minute. Drain well, squeezing out excess liquid. Pat dry with paper towels. Snip spinach with kitchen shears to coarsely chop; set aside. If using frozen spinach, drain well, squeezing out excess liquid.
In a large skillet cook onion and garlic in hot butter about 5 minutes. Stir in cream, pepper, salt, and nutmeg. Bring onion mixture to boiling; cook, uncovered, until cream begins to thicken. Add spinach. Simmer, uncovered, about 2 minutes or until thickened. Season to taste with additional salt and pepper.
Makes 4 servings.