In a small bowl, mix lemon zest, parsley and garlic. Cook fettuccine according to package directions; drain.
For sauce, in a large skillet, heat butter over medium-high heat. Add onion; cook and stir 2-3 minutes or until tender. Add garlic and lemon zest; cook 1 minute longer. Stir in cream, salt and pepper. Whisk in cream cheese until melted. Remove from heat; cool slightly. Stir in lemon juice.
Add pasta, tomatoes and parsley to skillet; toss to combine. Serve immediately with lemon zest mixture and, if desired, Parmesan cheese.