Chicken Cheddar Rice Bake

2 Tbs margarine
2 Tbs flour
2 chicken bullion cubes
1 cup milk
8 oz cubed cooked chicken
1 cup slice mushrooms
2 cups cold cooked rice
4 oz shredded cheddar cheese

Melt margarine, and blend in flour and bullion.  Gradually add milk, stirring until mixture comes to a boil.  Remove from heat.  Add chicken and mushrooms.  Put rice in a non-stick 8" pan.  Spoon chicken mixture over the rice and sprinkle evenly with cheese.  Bake at 350 degrees for 30 minutes.