Sauté shallots and mushrooms in 2 tablespoons butter until soft.  Add pistachios and pepper.  Cut pockets in thick portion of each breast lengthwise.  Place about ¼ cup pistachio mixture in each pocket; secure pockets with toothpicks.  Brown chicken on both sides in remaining butter.  Add broth and wine; bring to boil.  Cover and simmer about 10 minutes or until chicken is no longer pink.  Remove toothpicks; set chicken aside and keep warm.  Strain pan juices if desired; reduce by half.  Add cream and reduce to about ½ cup.  Stir in mustard.  Spoon over chicken breasts.  Garnish with chopped pistachios.