Cook fettuccine according to package directions. Drain and set aside. Meanwhile, separate caps and stems from shiitake mushrooms. (Reserve stems to use in stock or discard). Slice mushroom caps.
In a large skillet, cook onion and garlic in hot oil over medium-high heat until onion is tender but not brown, stirring occasionally. Add mushrooms and white pepper. Cook for 2 to 3 minutes or until mushrooms are tender and most of the liquid has evaporated, stirring occasionally. Stir in alfredo sauce; heat through.
Add fettuccine to skillet and toss with sauce until well coated. Transfer to a warm serving dish. Sprinkle with Parmesan cheese. Serve immediately.