How to Make It
Preheat the oven to 325°. Butter six 6-ounce ramekins and dust them with cheese. Set the prepared ramekins in a large baking dish.
In a small saucepan, melt the 4 tablespoons of butter. Over moderate heat, stir in the flour until incorporated, then gradually whisk in the milk until smooth and thick. Cook, whisking often, until no floury taste remains, about 5 minutes. Add the nutmeg and season with salt and pepper. Transfer to a bowl, press plastic wrap directly onto the surface and let stand until warm.
Meanwhile, in a large pot of boiling water, cook the spinach until just tender, about 1 minute. Drain and let cool slightly, then squeeze out as much water as possible. Transfer the spinach to a food processor and puree. Add the white sauce and process until blended. Add the eggs, egg yolk, 1/2 cup of cheese and 1 teaspoon of salt and process until smooth. Pour the sformato mixture into the prepared ramekins. Carefully pour enough hot water into the baking dish to reach halfway up the sides of the ramekins.
Cover the baking dish with foil and bake the sformati for about 1 hour and 5 minutes, until just set. Transfer the ramekins to a work surface; let stand for 5 minutes. Run a small, thin knife around the edge of the ramekins and invert the sformati onto plates. Drizzle with olive oil, sprinkle with cheese and serve.