This dish was inspired by my trips to the Tuscan countryside, where I tasted freshly pressed olive oil. Spinach is a great vehicle for olive oil; together, they create a light, versatile vegetable custard for any occasion.
This step can be done in advance.Preheat the oven to 375 degrees F. Grease 6 individual muffin liners or ramekins, place in a larger baking dish and set aside.In a small (3-quart) saucepan, melt the butter over low heat. Add the flour to the melted butter and mix until the flour is evenly absorbed. Continue to stir and cook the bechamel sauce base, until the flour begins to thicken and bind with the butter, about 1 to 2 minutes. Using a whisk, slowly add the cold milk, a little at a time, until all the milk has been incorporated. Bring the sauce to a simmer, and cook for 10 minutes. Remove from the heat, cover the surface of the bechamel sauce with plastic wrap, and allow to cool slightly.
Heat a large saute pan over medium heat and add a tablespoon of the olive oil.Add the spinach and sauté quickly until tender, about 1 minute. Season the spinach to taste, usually a scant 1/2 teaspoon, remove from the saute pan, spread out on paper towels and cool to room temperature. Squeeze the spinach gently to remove any excess water.Blend the remaining olive oil, béchamel sauce, parmesan cheese, 1 teaspoon salt, eggs and egg yolk, and spinach until very smooth. Fill the prepared liners 3/4 full with the batter. Pour hot water into the baking dish, filled to 2/3 of the way up the custards. Place the larger baking dish on the lower rack in the oven. Bake for 40 minutes. The custards are cooked when a toothpick or cake tester inserted into the center comes out clean.
Run a knife around the edge of the sformato and invert or unmold onto a small plate. Serve with your best-quality extra virgin olive oil and freshly grated Parmesan cheese.