Make the ragu: In a 5- to 6-qt. Dutch oven, melt the 6 Tbs. butter with the olive oil over medium heat. Add the celery, carrots, and onions and cook, stirring occasionally, until softened and lightly golden, about 15 minutes. Add the beef, pork, and pancetta and cook, breaking up the meats with a spoon and stirring often, until the meats lose their redness, 5 to 8 minutes. Stir in the wine, tomato puree, and tomato paste and simmer vigorously until the liquid is almost evaporated, 15 to 20 minutes. Add the beef or chicken broth and the milk, cover with the lid ajar, reduce the heat to low, and simmer gently until you have a rich, concentrated sauce, about 2-1/2 hours. Check every 30 minutes to see if more liquid is needed, adding more broth if necessary to prevent scorching. Season to taste with salt and pepper, if needed. Let cool. Skim off the fat from the top, if desired. You should have about 6 cups. Cook the noodles: Bring a large pot of salted water to a boil. Prepare a large bowl of ice water. Slip the noodles, two or three at a time, into the boiling water and cook them until they're tender and pale, 3 to 5 minutes (thinner noodles will cook more quickly). If using dried noodles, cook according to box directions. Carefully scoop the noodles out of the water with a large wire skimmer and slide them into the ice water to stop the cooking. When they're cool, layer them between clean dish towels until you're ready to assemble the lasagne (they'll keep this way for up to two hours). Make the cream sauce: In a 2-qt. saucepan, melt the butter over medium-low heat. Add the flour and cook, whisking constantly for 2 to 3 minutes. Do not let the mixture brown. Slowly whisk in the hot milk and bring just to a simmer, whisking frequently. Reduce the heat to low and cook, whisking often, until the sauce has thickened to a creamy, gravy-like consistency and no longer tastes of raw flour, 10 to 12 minutes. Remove from the heat and whisk in the salt, pepper, and nutmeg. If not using right away, transfer to a bowl and press a piece of plastic wrap directly on the surface of the sauce to keep a skin from forming. Plan to use the sauce within 30 minutes because it thickens if it's left to sit for too long. If that should happen, add a little warm milk and whisk well to thin it. Assemble the lasagna: Position a rack in the center of the oven and heat the oven to 350°F Choose a baking dish that's about 9x12 inches and 3 inches deep, or about 10x14 inches and 2 inches deep. Butter the baking dish. Spread 1/2 cup of ragu in a sparse layer on the bottom of the dish. Then cover with a slightly overlapping layer of cooked noodles, cutting them as needed to fill the gaps. Spread one-third of the remaining ragu (about 1-1/2 cups) over the first layer of noodles. Drizzle on one-third of the Basic Cream Sauce (about 1 cup) and spread it with a spatula or the back of a spoon. Sprinkle with 1/3 cup Parmigiano. Add a new layer of noodles, overlapping them slightly. Repeat the layers as instructed above, until all of the filling ingredients are used, to make a total of three layers (you may not need all the pasta). Dot the top with the butter cubes. Put the baking dish on a baking sheet and bake until heated through and bubbling at the edges, 45 to 50 minutes. Remove from the oven and let rest for 10 to 15 minutes before serving. Make Ahead Tips: You can prepare the ragu up to four days ahead and refrigerate it, or freeze it for up to one month. Reheat gently until the sauce is warm enough to spread for assembling the lasagne. The assembled, unbaked lasagne will keep for at least a day in the refrigerator. Alternatively, you can freeze it for up to four months (make sure you wrap the dish tightly with plastic). Thaw the lasagne in the fridge and bring it to room temperature before baking.