Fettuccine with Parsley Pesto and Walnuts

In a large pot of boiling salted water, cook pasta until al dente. Reserve 1/2 cup pasta water; drain pasta, and return to pot.

In a food processor, puree parsley, walnuts, Parmesan, garlic, lemon juice, and 2 tablespoons water until a paste forms. With machine running, add oil in a thin stream; process until very smooth, about 1 minute. Season pesto with salt and pepper.

To pasta in pot, add pesto, 1/4 cup pasta water, and spinach; toss to combine. Thin with pasta water as needed. Serve sprinkled with walnuts and Parmesan.