Fill a large saucepan about two-thirds full with cold water. Bring to a boil and add a small handful of salt. Cook the pasta until al dente (up to 22 minutes for pici; more like 10 minutes for fettuccine).
Meanwhile, heat half of the pesto over low heat in a large skillet. Drain the pasta, reserving about 1 cup of the cooking liquid, and turn it into the skillet. Mix well, adding more pesto (you may not need it all) and as much cooking water as needed for a saucy consistency; cook for a minute or two to blend the flavors.
Penne with Tuscan Kale and Walnut Pesto https://tavolatalk.com/pasta-with-cabbage-pasta-with-kale/