How to Make It
In a medium heavy skillet, heat the oil over medium heat until hot. Fry 1/4 cup of the sage leaves until crisp, about 1 minute. Transfer the sage leaves to paper towels to drain. Let the oil cool to room temperature.
In a food processor with the motor running, drop in the garlic and finely chop. Add 3/4 cup of the walnuts, the cheese, 1 teaspoon salt and 1/2 teaspoon pepper and pulse to finely chop. Add the parsley and the remaining 1/2 cup sage leaves and pulse to finely chop. With the motor running, add the cooled oil, blending to combine.
In a large pot of boiling salted water, cook the pasta until al dente. Reserve 3/4 cup of the pasta cooking water, then drain the pasta. Toss the pasta with the pesto and about half the reserved cooking water. Add more water for a looser consistency. Season the pasta to taste with salt and pepper, then serve sprinkled with the fried sage leaves and the remaining 1/4 cup walnuts.