Melt butter in a skillet 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.
