Whole-Wheat Spaghetti with Leeks and Hazelnuts

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Kosher salt

12 ounces whole-wheat spaghetti

4 tablespoons extra-virgin olive oil

3 large leeks, white and light-green parts only, thinly sliced

2 teaspoons sugar 

2 teaspoons balsamic vinegar (preferably aged)

1 small head radicchio, halved, cored and thinly sliced 

6 ounces creamy Italian cheese, such as fontina or taleggio, cubed

1/4 to 1/3 cup hazelnuts, toasted and chopped