Heat 2 tablespoons olive oil in a skillet over medium-low heat; add 3 sliced leeks (white and light green parts only) and season with salt and sugar. Cook, stirring occasionally, until tender, 10 minutes. Toss with 12 ounces cooked rigatoni, 1 cup pasta-cooking water, 2 tablespoons olive oil, 1 teaspoon sugar and 2 teaspoons balsamic vinegar. Add 1 small head sliced radicchio and 6 ounces cubed fontina; toss until the cheese melts. Top with chopped toasted hazelnuts.