Bring a large pot of salted water to a boil. Add the spaghetti and cook as the label directs. Meanwhile, heat a large skillet over medium-low heat and add 2 tablespoons olive oil. Scatter the leeks in the skillet and season with 1 teaspoon each salt and sugar. Cook, stirring occasionally, until tender, about 10 minutes. Drain the pasta, reserving about 1 cup of the cooking liquid. Add the pasta and the reserved cooking liquid to the skillet with the leeks. Add the remaining 2 tablespoons oil, 1 teaspoon sugar and the balsamic vinegar. Increase the heat to medium-high and add the radicchio and cheese. Toss until the cheese melts, 3 to 5 minutes. Season with salt and top with the hazelnuts.
Photograph by Antonis Achilleos
Recipe courtesy Food Network Magazine