For the chicken mousse: Puree chicken in a food processor until smooth. Add salt and pepper and the egg. Pulse just to combine and scrape the sides. While food processor is running, add cream gradually through the feed tube. Chill and reserve.
Wash and slice the mushrooms. In a hot saute pan, cook the mushrooms with butter. When mushrooms are brown, add shallots and herbs. Remove from the pan and chill.
Fold together mushrooms and chicken.
Lay out plastic wrap on a table. Down the center, spoon a 1-inch pile of the mushroom mixture. Roll the plastic into a log. Tie the ends with a string and tie into links. Poach in simmering water for 10 minutes. Shock the links in ice water. This can be done up to 3 days ahead.
To serve, remove the sausage from the plastic and grill or roast it until it is hot throughout (about 10 minutes). Slice the sausage and serve it with a tossed salad.