Boil the pasta in salted water for 2 minutes less than the time indicated on the package.
Cut the mushrooms into thin slices then fry them in a wide dry pan for about 2-3 minutes. Add the chopped sage, the bruised garlic cloves and then the oil and the butter and keep on frying for another 4-5 minutes. Season with salt.
When the pasta is done, transfer it into the pan along with 4-5 tablespoons of the water they were boiled in. Cook for about 2-3 minutes, until all the water is evaporated, then add the cream and keep cooking for another 3 minutes.
At the end, season with freshly chopped rosemary and ground black pepper.