Preheat oven to 180/Gas 4.
1) Lay walnuts on a baking tray and bake 7 to 10 minutes, until toasted and fragrant. Cook the pasta in a large pot of boiling salted water until al dente.
2) In a skillet, melt the butter over medium heat. Continue to heat and stir the butter until it turns light brown, about 4 minutes. Remove from the heat and add the sage leaves. Cook until crisped and the butter has turned a dark shade of brown.
3) Toss the butter and sage gently with the pasta, being careful not to break the sage leaves too much.
Divide pasta among serving bowls and top with the crushed toasted walnuts.