Instructions

Make the Bolognese sauce

• In a large Dutch over or other heavy-duty pot, melt 2 Tbs. of the butter with the oil over medium-high heat. Add the onion, carrot, and celery, and cook until softened, stirring occasionally, 4 to 5 minutes. Add the pancetta, and cook until the vegetables are caramelized and the pancetta is golden, 10 to 12 minutes.
• Over medium heat, add the beef and pork, and continue to cook, stirring continuously and breaking up the meat into small pieces, until the meat is cooked through and any liquid has evaporated, about 12 minutes.
• Whisk the tomato paste into the stock. Over low heat, add 1 cup of the stock to the meat in the pan and cook, stirring often until the pan is almost dry, about 30 minutes. Add the remaining stock 1 cup at a time, cooking until dry before adding the next, and ending wiht a dry pan, about 2 hours total. Add the chicken livers, if using, in the last 30 minutes of cooking. Season to taste with salt and pepper.
• Remove the pan from the heat, but while still warm, stir in the remaining 4 Tbs. butter one piece at a time. Season to taste. Cover and set aside until ready to use.

Make the béchamel sauce

• Heat the butter in a large saucepan over low heat, until foaming. When the foam subsides, add the flour, and cook, stirring, until a toasted, nutty aroma develops, but no browning, 2 to 3 minutes. Whisking constantly, add the milk gradually, 1 cup at a time, until the mixture is smooth before adding the next cup. Bring to a boil, stirring occasionally at first, then continually once boiling, until the mixture is thick and slightly reduced, 2 to 3 minutes. Add the nutmeg, and season to taste with salt and pepper. Set aside.

Assemble and bake the lasagna

• Position a rack in the center of the oven, and heat the oven to 375°F.
• Evenly spread 1/2 cup of the béchamel sauce in the bottom of a 9x13x2-inch slope-sided baking dish.
• You can assemble and bake the lasagna using uncooked sheets of fresh pasta, but for a finer finished texture, blanch the pasta before assembly. To blanch the pasta, straddle a large roasting pan over two burners, fill halfway with well-salted water, and bring to a low boil. Blanch the sheets of pasta in the water one at a time until slightly softened, about 1 minute. Using rubber-tipped tongs and the handle of a wooden spoon, carefully transfer each sheet to a large rimmed baking sheet lined with a clean kitchen towel, and pat it dry with paper towels. Transfer to the baking dish with the béchamel, laying it in the pan flat and fitting it into the corners.
• Spread about 1/2 cup of the béchamel over the pasta sheet and then about 1-1/4 cups of the meat sauce, and finally sprinkle with about 2 Tbs. of the grated cheese. Continue in this way, blanching the pasta sheets and then layering with béchamel, meat sauce, and grated cheese, to make five layers of each, ending with a layer of meat sauce and any remaining grated cheese. Note that the last sheet of pasta may be slightly above the edge of the dish. For this reason, be sure to put the final layer of béchamel and Bolognese in the center of the pasta sheet, spreading to leave a 1-1/2- to 2-inch border. Then gently press the top pasta sheet around the inside rim of the baking dish to form a slight trench. The pasta sheets will drape over the edges of the dish.
• Put the lasagna on a large rimmed baking sheet, and transfer to the oven. Bake until browned and bubbling in the center, 40 to 45 minutes. Tent with foil after the first 20 minutes of baking. Remove the foil for the last 5 minutes, until the pasta edges turn golden brown.