Melt butter in a frypan over medium heat. Saute mushrooms until tender. Reduce heat to low and stir in cream cheese until melted. Remove from heat.
Pound chicken breasts thin with a meat mallet. Spread with mushroom mixture and roll up. In a small bowl mix together brown sugar and Dijon mustard. Press mustard mixture onto chicken. Roll chicken in chopped nuts. Place in a baking dish.
Bake in preheated oven for 15 to 20 minutes, or until chicken is no longer pink and juices run clear.