Bring a large pot of water to a boil. Cook spaghetti according to package instructions. Reserve 1/2 cup pasta water, then drain.
In a blender, combine spinach, basil, olive oil, lemon juice, garlic, and salt. Blend until smooth and vibrant.
Return pasta to the pot and add the spinach sauce. Toss to coat evenly.
Add reserved pasta water gradually to loosen the sauce as needed.
Divide into bowls and top with cheese, walnuts, pepper, and lemon zest. Garnish with basil.