Blend the chicken breast and egg white in a food processor until the mixture is smooth. Transfer to a bowl and chill for 30 minutes.

Preheat the oven to 375 degrees F.

Place the bowl of chicken mixture over a bed of ice and slowly stir in the heavy cream. Pass the mixture through a sieve. Season with the nutmeg, salt, and cayenne and fold in the watercress.

Remove the skin and fat from the chicken breast halves. Using a sharp knife with a narrow blade, put a hole down the center of the breast half, lengthwise, starting from the wing joint. Fill a pastry tube with the mousse and, using a plain tip, pipe the mousse into the hole. Repeat with the remaining breast halves.

Place the chicken in a roasting pan and pour the chicken broth over it. Cover the pan with aluminum foil and bake for 45 to 50 minutes.

To make the sauce, in a saucepan, sauté the shallot in the butter until it is soft but not brown, 3 to 4 minutes. Deglaze the pan with the wine, and reduce the mixture to 1/4 cup over high heat. Add the chicken stock and reduce to 1/2 cup.

Add the cream, bring to a boil, and simmer for 3 to 4 minutes. Add the watercress and tomatoes and salt and pepper.

Remove the chicken from the broth and place on warm individual plates. Surround the chicken breast with sauce and garnish with watercress sprigs.