1. Preheat the oven to 350°. Lightly butter and flour a 1½-quart ring mold. Melt the 2 tablespoons of butter in a heavy 3- or 4-quart saucepan, remove from heat and stir in the 2 tablespoons of flour.
2. Then pour in the milk and stir with a whisk until the flour is partially dissolved. Return the pan to low heat, and cook, stirring constantly, until the sauce comes almost to a boil and is thick and smooth.
3. Remove from heat and beat in the egg yolks, one at a time, whisking until each one is thoroughly blended in before adding the next. Stir in the spinach, grated onion, salt and pepper, and allow the mixture to cool slightly.
4. Beat the egg whites with a wire whisk or rotary beater-and in an unlined copper bowl if possible-until they are stiff enough to form unwavering peaks on the beater when it is lifted from the bowl.
5. Stir a large spoonful of egg white into the spinach mixture and then gently fold in the remaining egg whites. Ladle the mixture into the mold. Place the mold in a baking pan and pour enough boiling water into the baking pan to reach 2/3 of the way up the sides of the mold. Bake on the middle shelf of the oven for 30 minutes, or until it is firm to the touch.
6. To turn out the spinach ring, wipe dry the outside of the mold and run a knife around its inside surfaces. Place a serving plate upside down over the top of the mold, and, firmly grasping plate and mold together, invert the two. Rap them sharply on a table, and the spinach ring should slide out easily onto the plate. Serve at once.
