To Make Tortelloni Al Forno: Preheat oven to 425 F. Bring 6 quarts of water to boil and add 2 tablespoons salt.
Heat oil in a 12 to 14-inch saute pan until smoking and add onions. Cook onions until softened but not brown, about 1 to 2 minutes. Add radicchio and cook until softened and lightly gray in color, about 5 to 6 minutes. Remove from heat and allow to cool.
Meanwhile, drop tortellini into boiling water and cook until tender. Gently drain and place in a large mixing bowl. Add 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.