Fettuccine with Parsley Pesto and Walnuts

I love traditional basil pesto, but love this for a little change of pace. Recipe from Everyday Foods.
*3/4 lb. fettuccine
*2 c. packed fresh parsley
*1/4 c. walnuts, plus more chopped for serving
*1/4 c. grated Parmesan, plus more for serving
*1 garlic clove
*1 TBSP. fresh lemon juice
*1/4 c. olive oil
*5 oz. baby spinach

In a large pot of boiling salted water, cook pasta until al dente.  Reserve 1/2 c. 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 (and/or red pepper flakes). To pasta in pot, add pesto, 1/4 c. pasta water, and spinach; toss to combine.  Thin with pasta water as needed.  Serve sprinkled with walnuts and Parmesan.  This is delicious as a main meal or a side with a grilled steak or pork chop.