Add pesto to a very large skillet. Warm over low heat, stirring frequently.
In a large pot, bring 4 to 6 quarts of water to a rolling boil; add salt to taste and the Fettuccine; stir gently.
Cook pasta according to package directions, adding the kale during the last 2 minutes of cooking. Remove from heat and drain well, reserving 1/2 cup pasta cooking water.
Stir the pasta into the pesto and warm through on low. If needed, stir in up to 1/2 cup reserved pasta cooking water to reach desired consistency. Serve, topped with tomatoes, walnuts, and lemon zest.
Customize it: For a bit of added protein use baby spinach instead of the kale, or top with white beans.
