Cook the fettuccine based on package instructions. Set aside.
Make the sauce by melting butter in a pan. Add the olive oil.
Sauté the mushrooms until soft.
Combine Knorr Cream of mushroom and water. Mix well. Pour the mixture into the pan and add the rosemary. Cook for 10 minutes using the lowest heat setting.
Pour the heavy cream and add truffle oil. Season with ground black pepper and parmesan cheese.
Toss-in the fettuccine making sure that it completely gets coated with the sauce.
Transfer to a serving plate. Sprinkle parsley and top with more parmesan.