In a skillet over medium heat, sauté the celery, onion, and carrot with a drizzle of olive oil and the bay leaf until golden brown. Add the ground beef, pork, and pancetta, and cook until the meat starts to brown. Season with salt and pepper. Stir in the red wine, cook until the wine evaporates completely, then lower the heat and add the tomato paste. Add 2 cups of water, or enough water to cover the meat mixture, and cook slowly over low heat for at least 30 minutes. Discard the bay leaf.

While the meat sauce is cooking, prepare the béchamel sauce. Melt the butter in a heavy-bottomed pan, add the flour, and whisk together for 3-4 minutes over low heat until smooth. Heat the milk in a separate pan, then add it to the roux, pouring it in a slow stream. Adjust the salt, season with a bit of nutmeg, and continue cooking, whisking constantly to avoid the formation of lumps until the sauce is thick and creamy. If the béchamel is too thick, add a bit of milk. If it is too thin, let it cook for a few additional minutes.

Preheat the oven to 180°C. Butter a 9-by-13-inch baking dish and arrange the first layer of lasagna lengthwise. Evenly spread a layer of meat sauce over it, followed by the béchamel sauce and a generous handful of grated Parmigiano cheese. Repeat this process, layering the lasagna sheets, meat sauce, and béchamel sauce until you have used all the ingredients. Finish with a layer of béchamel sauce, a generous dusting of cheese, and a few pieces of butter.

Bake the lasagna for 25 to 30 minutes, or until golden brown. Let it rest for 5 minutes before serving.