3 tablespoons unsalted butter, plus more for greasing the dish 2 tablespoons grated Parmesan cheese 3 tablespoons flour 1 cup milk ¼ teaspoon ground black pepper ½ teaspoon salt 3 egg yolks at room temperature 1 cup shredded Gruyère cheese 2 tablespoon minced chives 4 egg whites at room temperature Preheat oven to 400º
