3 tablespoons (45g) unsalted butter, plus more for greasing the dish 2 tablespoons grated Parmigiano-Reggiano or Grana Padano cheese, for dusting the dish 3 1/2 tablespoons (1 ounce; 28g) all-purpose flour  1 cup (235ml) whole milk Kosher salt and freshly ground black pepper 1 teaspoon (5ml) Dijon mustard  1 pinch cayenne pepper or two dashes hot sauce 4 large egg yolks 5 large cold egg whites 1/2 teaspoon cream of tartar  3 ounces (85g) freshly grated Gruyère or other semi-firm cheese, such…