Heat a small amount of oil in a saucepan and fry the sage leaves, a few at a time, for a few seconds. Drain them on paper towels. Toast the walnuts in a dry pan. Melt the butter in a pan over low heat and let it brown to a hazelnut color. Meanwhile, cook the ravioli in lightly salted boiling water, then drain with a slotted spoon and transfer them directly into the pan with the butter. Toss, adding a few spoonfuls of pasta cooking water and mixing gently. Finish by sprinkling with roughly chopped walnuts, fried sage leaves, and pepper to taste.