Place a large pot of water over high heat to boil. When the water reaches a bubble, salt it and drop the pasta. Cook just shy of al dente according to package directions. Just before draining, reserve a large mugful of the starchy cooking water. Drain pasta and reserve
While the pasta is cooking, combine half of the spinach, the nuts, stock, Parmigiano Reggiano, nutmeg, lemon zest and juice, some salt and pepper in the bowl of a food processor. Pulse the machine to chop everything up then stream in about 1/4 cup EVOO (eyeball it) with the machine running. Reserve.