Heat 3 cups milk in a small saucepan over low heat. Melt 6 tablespoons unsalted butter in a medium saucepan over medium heat. When the butter foams, add 5 tablespoons flour. Lower the heat to medium-low and stirring, cook about 2 minutes, without letting the flour burn.
Remove the saucepan from the heat and pour in the milk all at once. Mix enthusiastically to prevent lumps. Place saucepan back on low heat, season with salt to taste, and cook gently,stirring constantly until the sauce has a medium-thick consistency. This takes around 3 to 5 minutes. If the sauce is too thin, cook a bit longer. If too thick add a bit more milk. Cover the pan and set aside until ready to use.
Preheat oven to 375 degrees F. Generously butter a 9 by 13-inch baking dish. Bring a large pot of water to a boil. Add about 1 tablespoon salt and 1 pound of penne. Cook uncovered over high heat 7 to 8 minutes. The penne shouldn't be totally cooked because it will cook more in the oven. Drain the pasta and place in the baking dish.
Stir in 1/4 teaspoon freshly ground nutmeg to the bechamel.
Add the bechamel to the pasta and mix in well.
Sprinkle the penne with 3/4 cup freshly grated parmigiano. Sometime I use 1/2 cup grated Vella dry Jack from Sonoma, CA and 1/4 cup pecorino romano. Dot with 3 tablespoons unsalted butter that has been cut into small pieces. Bake until the cheese melts and the penne has a golden color, around 15 minutes or so. Serve hot.