Boil the potatoes whole in water until very tender. Drain and, while still warm, peel, and run through a ricer or food mill. Place the potatoes in a large bowl and add the egg, a pinch of salt, and enough flour to make a dough that is the same consistency of gnocchi dough. Knead the dough for 5 minutes, then let rest for 30 minutes.
In a large saute pan, heat the olive oil over high heat, then add the onion, veal , and sausage, and brown over medium-high heat until it is nicely browned. Add the spinach, season with salt and pepper and cook over high heat until the spinach is well cooked and most of the liquid has evaporated. Let cool.
Roll the pasta dough out on a pasta machine down to the thinnest setting, and cut 3-inch squares from the dough. Place a scant tablespoon of the filling in the middle of 1/2 of the squares, and top them with the other squares, pressing the edges together to seal. Set aside.
Bring 6 quarts of water to a boil and add 2 tablespoons salt. Cook the ravioli in the boiling water until they are tender and cooked through, about 4 minutes. Drain and place in a serving bowl. Drizzle the butter over, top with grated Parmigiano-Reggiano and serve.