10 ounces farro spaghetti or whole-wheat spaghetti
Pulse 5 cups watercress, pine nuts, lemon zest, and garlic in a food processor until coarsely chopped. Add oil and pulse to combine. Season with salt and red-pepper flakes.
Meanwhile, bring a large pot of generously salted water to a boil. Add pasta and cook until al dente. Reserve 1 cup cooking water; drain pasta. Return pasta to pot and stir in pesto. Add enough reserved pasta water, a small amount at a time, to coat pasta and make a light sauce. Season with salt and top with remaining watercress.