Preparation

• Heat the oven to 350°F. Butter a medium (1-1/2- quart) shallow baking dish.
• Fill a large pot about halfway with lightly salted water and bring to a boil over high heat. Add the spinach in large handfuls, adding more as it wilts. When all the spinach is in the pot, blanch for 1 minute. Drain in a colander, running cold water over the spinach to cool it. Squeeze the cooled spinach to remove as much liquid as possible and then chop it coarsely.
• Heat the milk or broth in a small saucepan or in the microwave until hot. Set a medium saucepan over medium heat and melt the butter. Add the onion and cook until translucent and softened but not colored, about 5 minutes. Stir in the flour, salt, pepper, and cayenne and cook, stirring constantly, for 2 minutes. Slowly stir in the milk or broth, raise the heat to medium, and cook, stirring frequently, until the sauce bubbles and thickens, about 5 minutes.
• When the sauce is ready, add 1 cup of the Parmigiano and stir until the cheese is melted and thoroughly combined. Take the pan off the heat and stir in the chopped spinach, mixing well. Taste and season with more salt if necessary. Scrape into the buttered baking dish, sprinkle the remaining 1/4 cup Parmigiano on top, and bake until the gratin is bubbly and the top is nicely browned, 35 to 40 minutes. Let it cool for a few minutes before serving.