Place the parsley, sage, Parmigiano-Reggiano, walnut pieces, garlic and pepper in a food processor fitted with a chopping blade, or in a mini food processor, or a wide blender.
Pulse four or five times until coarsely chopped, scraping down the sides of the bowl as necessary.
With the food processor or blender running, drizzle in the olive oil until a thick sauce forms. (The consistency of pesto is a matter of preference. If you prefer a coarser sauce, pulse only a couple of times before you add the oil, just to chop everything roughly. For a smoother pesto, pulse up to ten times before you add the oil.)
Toss the cooked pasta with the sage pesto in a large bowl, sprinkle with the grated lemon zest and serve immediately.