place chicken in large zip-loc bag or bowl and pour over 2 cups buttermilk. Let set at room temp until chicken is at room temp at least 1 hour
Mix panko, flour salt and pepper together
heat 2 - 3 inches of oil in large skillet and pre-heat oven to 400 degree
remove chicken 1 breast at a time and coat with panko/flour and remove to wax paper covered plate or baking sheet when all breasts are coated add gently to oil and fry until both sides are golden brown and juices run clear (about 20 mins depending on thickness of your chicken breast) while chicken is frying line a dish with paper towels to drain chicken on when removed from pan.
sautee mushrooms until half cooked (cooked but still firm)
when chicken is done drain and put on a baking sheet top with mushrooms and 1 to 2 slices swiss cheese. Bake in 400 degree oven until cheese is melted.