Briefly toast the flour in a casserole, and then add half of the milk a little at a time, stirring constantly to avoid lumps forming. Allow the sauce to thicken over low heat, continuing to stir, and slowly adding the remaining milk. When the mixture has reached the consistency of liquid cream, season it with salt and some freshly grated nutmeg, then blend in two thirds of the grated cheese. Mix well and continue to heat for 2 minutes before turning off: the sauce should remain fairly fluid. Cook the pasta in abundant salted water, drain when “al dente” and pour into the pan containing the sauce. Add a little of their cooking liquid, some flakes of soft butter and the sage leaves. Mix over the heat for 2 minutes, then serve sprinkled with the remaining Grana-style cheese.