Cook and stir mushrooms in 2 Tbsp. margarine in medium saucepan on medium-high heat 2 min. or until tender. Stir in water, 3 Tbsp. mustard and 1/4 tsp. thyme. Bring to boil.
Remove from heat. Stir in stuffing mix; spoon loosely into body cavity of chicken. Tie legs to tail. Twist wing tips under back.
Place chicken, breast-side up, on wire rack in shallow roasting pan. Melt remaining margarine; mix with remaining mustard and thyme.
Brush over chicken; cover loosely with foil.
Bake 1 hour 45 min. or until chicken is done (165F), removing foil for the last 45 min.