Preheat Oven To 450 Degrees F (230 Degrees C). 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.