Put your watercress, pine nuts, lemon zest, and garlic in a food processor and pulse until coarsely chopped. Add oil and pulse again to combine. Season with salt and red-pepper flakes.
Boil a large pot of salted water. Add pasta and cook until al dente. Set aside 1 cup of cooking water; drain pasta. Return the pasta to pot and stir in pesto. Add a small amount at a time of your reserved pasta water, to coat pasta and make a light sauce. Season with salt and top with remaining watercress.