Make the tomato and béchamel sauces first. To save time you can make the tomato sauce a couple of days ahead. Béchamel sauce is best the day it's made and I like to make it after browning the meat. Set it aside, off the heat, while you assemble the rest of the ingredients.
In a large skillet, heat the butter over high heat until it bubbles. Add the ground pork and veal and cook over high heat until the meat is browned. Drain the excess fat and stir in the Parmigiano, the flour and the milk and reduce heat to a simmer.
Cook for 10 minutes, stirring regularly, then remove from heat and allow to cool. Season with salt, pepper and nutmeg and stir in the egg and spinach. Set aside.
Preheat the oven to 350 degrees F.
Cook cannelloni according to package directions but be sure to subtract 3 minutes from the cooking time. This will ensure the pasta is al dente and won't turn to mush when you bake it.
When the pasta is done, strain and let cool. When you can comfortable hold the tubes, use a spoon to stuff the filling into each tube.
If you're using fresh pasta sheets, cut the sheets into rectangles measuring 4 inches by 6 inches.
Divide the meat filling evenly among the rectangles and roll the pasta around the filling, jelly-roll style making 14 to 16 canneloni.
In an 8 or 9-inch square, oven-proof casserole, spoon half the tomato sauce across the bottom in an even layer. Carefully lay each stuffed pasta in the casserole so that they are in a single layer. Top with the remaining tomato sauce followed by the bechamel sauce.
Finish with the freshly grated Parmigiano-Reggiano and bake in the oven for 25 minutes, until the sauces and cheese are bubbling and the edges of the pasta are crispy and browned. Serve immediately.