Place chicken in resealable plastic bag. Pour italian dressing over chicken. Seal bag and marinate in refrigerator for 4-24hrs. Turning the bag occasionally.
When ready to cook, drain and discard marinade. Place chicken on broiler pan. Broil for about 13 minutes or until chicken is no longer pink in the center.
In medium saucepan cook mushrooms and garlic in margarine over medium high heat until mushrooms are tender. Stir in flour and pepper. Stir in milk all at once. Cook mushroom mixture until thick & bubbly. Add cheese and stir until melted.
When ready to serve spoon hot cooked rice onto serving plates. Arrange chicken breast on top of rice. Laddle mushroom sauce over it all.