Place mussels in a large bowl, and cover with cold water. Sprinkle with cornmeal; let stand 30 minutes. Drain and rinse.
Combine mushrooms and next 12 ingredients (mushrooms through garlic) in a large Dutch oven; bring to a boil. Reduce heat, and simmer, uncovered, 3 minutes. Add mussels; cover and cook over medium-high heat 3 minutes or until mussels open. Discard any unopened shells.
Spoon the fettuccine into shallow bowls; top with mushroom sauce and mussels. Sprinkle with parsley.