For the pasta: In a large mixing bowl, mash the cheese until soft. Add the scallions, eggs, nutmeg, cheese, salt and pepper. And stir well. Refrigerate until firm, 30 minutes or more. Using a pasta machine, roll out the pasta to the thinnest setting and then cut the sheets into 2-inch squares. Place 1 teaspoon of the filling in the center of each 2-inch square. Fold the opposite corners together to form a triangle and press the edges together firmly to seal. Bring the long points of the triangle together and join with firm finger pressure. Continue filling and shaping tortellini until all the pasta and filling are used.
Preheat the oven to 425 degrees F.
Bring about 6 quarts of water to a boil and add about 2 tablespoons salt. Heat the olive oil in a 12-inch saute pan until almost smoking and add the onion. Cook until softened but not brown, 1 to 2 minutes. Add the radicchio and cook until softened and lightly gray in color, 5 to 6 minutes. Remove from the heat and allow to cool. Meanwhile, drop the tortellini into the boiling water and cook until tender, about 6 to 7 minutes. Gently drain and place in a large mixing bowl. Add the cooled radicchio mixture, balsamella, and half the grated cheese. Gently stir to mix well. Divide among 4 buttered gratin dishes, sprinkle with the remaining cheese, and bake until bubbly and crusty on top, about 20 minutes. Serve immediately.
Mound 3 1/2 cups of the flour in the center of a large wooden cutting board. Make a well in the middle of the flour and add the eggs and olive oil. Using a fork, beat together the eggs and oil and bring to incorporate the flour starting with the inner rim of the well. As you expand the well, keep pushing the flour up from the base of the mound to retain the well shape. Do not worry that this initial phase looks messy. The dough will come together when half of the flour is incorporated. Start kneading the dough with both hands, using the palms of your hands. Once you have a cohesive mass, remove the dough from the board and scrape up and discard any leftover crusty bits. Lightly re-flour the board and continue kneading for 6 more minutes. The dough should be elastic and a little sticky. Wrap the dough in plastic and allow to rest for 30 minutes at room temperature. Roll or shape as desired.
In a medium saucepan, heat the butter until melted. Add the flour and whisk until smooth. Cook over medium heat until golden brown, about 6 to 7 minutes. In a separate pan, heat the milk until just about to boil. Add the milk to the butter mixture 1 cup at a time, whisking continuously until very smooth, and bring to a boil. Cook 30 seconds longer and remove from the heat. Season with salt and nutmeg.
Recipe from Simple Italian Food-Recipes from My Two Villages, by Mario Batali, published by Potter, 1998