Meanwhile, heat the EVOO in a large skillet over medium-high heat. Brown the sausage and crumble to very small pieces. Then add the veal, brown and crumble. Add the sage, garlic, onions and some salt and pepper, and cook until tender. Separate the spinach with your fingertips and add to the skillet. Stir to heat through, then deglaze with the wine and remove from the heat.
In a saucepot over medium heat, melt the butter, whisk in the flour and bring to a bubble. Whisk in the milk and season with salt, pepper and nutmeg to taste. Thicken the sauce to coat the back of a spoon. Taste to adjust the seasoning and remove a ladle of sauce to combine with the egg yolks to temper them. Add the egg yolks to the sauce and stir in half of the fontina cheese and Parmigiano-Reggiano. Reduce the heat to low, or cover to keep warm.
Salt the water, and cook the pasta to a chewy al dente, 6 to 7 minutes. Then drain and cool. Fill the tubes or shells with the meat filling using small spoon. Cover the bottom of the baking dish with one-quarter of the sauce, then arrange the shells and cover with the remaining sauce, then top with the remaining cheese. Refrigerate or freeze for a make-ahead meal.
Bake from room temperature at 375 degrees F until brown and bubbly, 30 to 40 minutes.