Soak porcini mushrooms in cold water for about 25 minutes. Brush, clean and quarter the mushrooms. Peel garlic and onion and cut into fine cubes. Drain the porcini mushrooms.
Wash the sage and dab dry. Put 2 stems aside for garnishing. Cut the rest into fine strips. Prepare pasta in boiling salted water according to package instructions. Heat oil in a pan and fry mushrooms and bacon for about 2 minutes.
Add porcini mushrooms, sage, onion and garlic and continue to fry for about 3 minutes. In the meantime chop the walnuts roughly. Remove the pan from the heat, fold in the walnuts and 2 tablespoons of ricotta, season with salt and pepper.
Spread the pasta in a wide casserole dish. Add the ricotta-mushroom mixture to the pasta. Sprinkle with 30 g of gorgonzola and bake under the grill of the oven for about 3 minutes until golden brown. Bring milk to the boil in a small pot.
Melt the rest of the cheese in it, season with salt, pepper and sugar. Simmer for about 1 minute. Arrange noodles on preheated plates, drizzle with sauce and garnish with sage leaves.