Linguine with Leeks, Radicchio, and Walnut Pesto

8 ounces linguine
4 tablespoons extra-virgin olive oil, divided
4 cups thinly sliced leeks (including some dark green parts)
1/2 cup (packed) fresh Italian parsley leaves
1/4 cup grated Parmesan cheese plus shaved Parmesan for garnish
1/4 cup walnut pieces (about 1 ounce) plus additional for garnish
2 teaspoons fresh lemon juice
2 cups thinly sliced radicchio

1. Cook pasta in large pot of boiling salted water until tender but still firm to bite, stirring occasionally.

2. Heat 1 tablespoon oil in large nonstick skillet over medium high heat. Add leeks; season with salt and pepper. Cover; cook until tender and beginning to brown, stirring occasionally, about 10 minutes.

3. Puree parsley, 1/4 cup Parmesan, 1/4 cup walnuts, lemon juice, and 3 tablespoons oil in mini processor until coarse puree forms. Season pesto with salt and pepper. 

4. Drain pasta, reserving 1 cup cooking liquid. Add pasta, pesto, and radicchio to leeks; toss, adding cooking liquid by tablespoonfuls if dry. Garnish with walnuts and shaved Parmesan.

Source: Epicurious