Preheat oven to 400 degrees F and butter two 1/3- to 1/2-cup ramekins.
Discard coarse stems from spinach.
In a 2- to 3-quart saucepan of boiling salted water cook spinach 2 minutes and drain in a sieve, pressing with back of a large spoon to remove as much water as possible.
Finely chop spinach.
In a small nonstick skillet cook onion in butter over moderately low heat, stirring, until softened. Stir in spinach, cream, nutmeg, and salt and pepper to taste and cook, stirring, until hot.
Divide creamed spinach between ramekins and break an egg into each.
On a baking sheet bake eggs in upper third of oven until whites are cooked through, or until desired doneness, about 12 minutes. (The yolks will not be fully cooked, which may be of concern if there is a problem with salmonella in your area.)
Season eggs with salt and pepper and serve immediately.