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 425ºF.
Bring about 6 quarts of water to a boil and add about 2 tablespoons salt. Heat the olive oil in a 12 inch sauté 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.
Yield: 4 servings