FOR MORNAY SAUCE: Melt the butter in a heavy-bottomed saucepan over medium-low heat. Sprinkle the flour over the butter and cook for 1-2 minutes without allowing it to color, stirring constantly with a wooden spoon. Remove the pan from the heat and slowly add the milk, whisking or beating vigorously to avoid lumps. Return to medium heat and bring to a boil, stirring constantly. Simmer for 3-4 minutes, or until the sauce coats the back of a spoon. Stir in the nutmeg, then remove from the heat. Set aside, covered, and keep warm.
In a large skillet, melt the butter over low heat and add the spinach. Cook for about 5-8 minutes, or until dry. Set aside and keep warm.
Whisk the cheese into the mornay sauce, then whisk in the egg yolks. Season to taste, with salt and pepper. Place over low heat and mix until the cheese is melted, then heat until very hot but not boiling. Set aside, cover the surface with a piece of waxed paper and keep warm.
FOR EGGS: Half-fill a large skillet with water and bring to a boil over high heat. Reduce the temperature to low and add the vinegar. The water should be barely simmering.
Crack the eggs one at a time into a cup or bowl and carefully slide into the vinegared water two or three at a time. Cook for 2-3 minutes, or until the egg whites are firm but not hard. Very gently remove, using a slotted spoon and drain thoroughly.
Divide the cooked spinach evenly among four warmed plates. Place two poached eggs in the center of each mound of spinach and cover with the hot mornay sauce. Serve immediately.
CHEF'S TIP: Mornay sauce is a bechamel, or white sauce, flavored with cheese and enriched with egg yolks.
Recipe reprinted by permission of Tuttle Publishing/Periplus Editions. All rights reserved.