Heat a large dry skillet over medium heat. Add the chopped walnuts and toast for 3-5 minutes, stirring frequently, until fragrant and lightly browned. Be careful not to burn them. Remove from skillet and set aside.

Bring a large pot of heavily salted water to a rolling boil. Add the fettuccine and cook according to package directions until al dente. Before draining, reserve about 1 cup of the pasta cooking water. Drain the pasta and set aside.

While the pasta cooks, return the large skillet (no need to clean) to medium-low heat. Add the extra virgin olive oil. Once shimmering, add the minced garlic and red pepper flakes (if using). Sauté for 1-2 minutes until fragrant, being careful not to brown the garlic.

Remove the skillet from the heat. Add the finely chopped parsley, lemon zest, and toasted walnuts to the skillet. Stir to combine with the garlic-infused olive oil.

Add the drained fettuccine directly into the skillet with the walnut-parsley mixture. Add about 1/2 cup of the reserved pasta water. Toss vigorously, using tongs, until the pasta is well coated and a light sauce forms. Add more pasta water, a tablespoon at a time, if needed to achieve a creamy consistency.

Season with salt and freshly ground black pepper to taste. Squeeze a little fresh lemon juice over the top, if desired. Serve immediately, garnished with extra fresh parsley and a generous sprinkle of grated Parmesan cheese.