For the ragù: In a very large heavy-bottom pan, heat the olive oil. Add the onions and garlic, and over medium heat simmer for about 5 minutes, till the vegetables are soft, but not completely cooked. If you’re going to use carrots and celery, add them together with the onions. Add the bacon, veal and pork to the vegetables and brown them over medium to high heat, stirring to break up the big pieces of meat. When the meat is brown, add the paprika and the wine and give the mixture a good stir. Add the chopped tomatoes and the tomato puree. Season with salt and pepper, and simmer over low heat for 1 hour or more. Before removing from heat, add the chopped basil and stir well. Taste to check if it needs a little more seasoning. Meanwhile you can prepare the béchamel sauce.In a medium saucepan heat the butter over medium-low heat until melted. Add the flour and stir until smooth. Over medium heat, cook until the mixture gets a light, golden sandy color, about 5 to 6 minutes. Heat the milk in a separate pan until it’s just about to boil. Add the hot milk to the butter mixture 1 cup at a time, whisking continuously until very smooth. Bring to a boil. Cook for 6-7 minutes, stirring constantly, and then remove from heat. Season with salt and nutmeg, and set aside until ready to use. If you are not going to use the béchamel immediately, transfer it to a bowl and place plastic wrap directly onto the surface, to prevent forming a skin on top. Leave a bit of space around the edges for the steam to escape while it cools. When the ragù is ready you can assemble the lasagne. Preheat the oven to 190C degrees. Butter the sides of an ovenproof pan (25 x 35 cm). Spread a ladle of béchamel on the bottom of the pan, cover it with lasagna sheets, then distribute evenly some of the ragù, cover with béchamel and sprinkle some grated Parmesan cheese. Continue in that order, making 4 to 5 layers of pasta, finishing with béchamel and a good handful of the Parmesan cheese sprinkled over the top. Bake in the oven for 25-30 minutes, until the top is golden brown and bubbling. Remove from the oven; allow to cool for 10-15 minutes, slice and serve.