1. Place dried porcini in a small bowl; cover with with hot water. Let sit 30 minutes.
2. Meanwhile, brush dirt off mushrooms (wash quickly under cool running water if needed). Trim stem ends and discard; cut into thin slices.
3. Heat oil and butter in large nonstick skillet over medium-high heat. Add mushrooms and cook, stirring frequently, until mushrooms start to color, about 8 to 10 minutes. Drain porcini well, reserving liquid. Chop porcini and add to skillet along with rosemary and pepper. Cook another minute.
4. Add broth and reserved porcini liquid; bring to high heat and cook 10 minutes or until liquid starts to reduce. Add cream, reduce to medium-high and cook another 5 to 10 minutes or until sauce begins to thicken. Remove from heat; stir in cheese.
5. Cook fettuccini in boiling salted water until tender but still firm. Reheat sauce as needed over low heat. Drain fettuccini and return to pot. Add sauce and toss to combine. Serve with extra parmesan.Do-ahead tips:
6. Mushrooms can be sliced a day ahead and refrigerated; sauce can be made up to 5 hours ahead and refrigerated.