Cook the penne in boiling salted water for 12-15 minutes. In the meantime peel and finely dice the onion. Heat the fat in a pot, fry the onions in it until transparent and deglaze with cream. Bring to the boil and simmer at low heat for 2-3 minutes. Wash the sage, dab dry and cut into fine strips, except for something for garnishing. Dice gorgonzola roughly.
Add sage and stock to the sauce. Add the sauce thickener while stirring, add gorgonzola and bring to the boil again briefly. Season to taste with salt and pepper. Chop the walnuts. Pour the pasta onto a sieve, drain and arrange on plates with the sauce. Sprinkle with walnuts and serve garnished with sage