Fettuccine with Truffle Mushroom Sauce Recipe

1 tablespoon olive oil

1/2 cup butter

1 cup fresh portobello mushrooms, sliced

1 cup fresh button mushrooms, sliced

1 cup fresh shiitake mushrooms, sliced

1/2 cup heavy cream

1 sprig fresh rosemary, or 1/4 teaspoon dried rosemary, crushed

1/2 teaspoon salt

1/4 teaspoon ground black pepper

1/2 teaspoon white truffle oil

250 grams fettuccine, cooked according to package directions

1/3 cup parmesan cheese, grated

3 tablespoons flat-leaf parsley, chopped