Put walnut kernels, bread and garlic into a mixer or a food processor and pulse a few times.* Add the Parmigiano, salt and pepper and pulse again while adding the milk and olive oil until you get the desired consistency. Transfer the sauce in a large bowl.
Heat peanut oil in a small skillet over medium-high heat until hot. Fry 6-8 sage leaves at a time until crisp, for only 2-3 seconds. Transfer with a fork to paper towels.
Cook the pasta according to package instructions and drain, reserving ¼ cup of the pasta cooking water. Add the fettuccine to the bowl with the walnut sauce, adding a little of the pasta cooking water to loosen the sauce if needed and mix well.
Divide fettuccine among 2 plates, sprinkle with fried sage leaves and serve.
*For a more delicate sauce peel the walnut kernels after boiling them for 5 minutes.