In a 4- to 5-quart pan, combine 2 1/2 cups chicken broth, wine, whipping cream, lemon peel, fennel, 1/2 teaspoon pepper, and chile flakes to taste. Bring to a boil over high heat and boil, uncovered, 2 to 3 minutes.
Meanwhile, with a sharp knife, trim rind from brie and finely chop (rind adds flavor, but discard if you like). Cut brie into about 1/2-inch chunks.
Add fettuccine to pan, pushing it down into boiling liquid. Simmer, uncovered, stirring often, until pasta is tender to bite, 8 to 10 minutes.
Add chopped brie rind (if using) and chicken and stir to blend. Stir in chunks of brie. Remove pasta from heat and let stand, uncovered, 5 to 8 minutes, stirring often to make sure all the cheese melts (the rind won't melt).
If you want saucier pasta, add up to 1 cup more broth and stir over high heat until steaming.
With tongs or two forks, lift pasta into wide soup bowls; spoon remaining sauce over the top. Sprinkle with parmesan and more pepper to taste, and top with watercress leaves.