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.
Note: Tasty mushrooms:
Since mushrooms release liquid as they cook, it's important to get the process right, or the results can be pale and watery instead of deeply browned. The trick is high heat.
To prep: Trim discolored stem ends from 1 pound mushrooms. Either wipe the mushrooms clean or rinse briefly and pat dry, then thinly slice.
To cook: In a 12-inch frying pan over high heat, melt 2 tablespoons butter. Add mushrooms and stir until all the moisture has evaporated and mushrooms are browned, 8 to 10 minutes.
To serve: Season with salt, pepper, and fresh herbs, then use sautéed mushrooms to liven up anything from chicken breasts to eggs Benedict. Mix with chopped tomatoes for a bruschetta topping, toss into a spinach-feta salad, or stir into polenta.