Preheat the grill. Line a shallow baking tray or dish with aluminium foil, and place chicken on top. Grill chicken breasts for 25 to 35 minutes or until they are cooked through and the juices run clear. Turning the chicken over at about 15 minutes.
Meanwhile, cook mushrooms with salt and pepper in a medium frying pan (covered) over high heat until their juices run. Then lower heat, remove cover and continue cooking until liquid has evaporated. Add mushrooms to chicken and sprinkle grated cheese on top.
Grill for a further 8 to 10 minutes, until the cheese is golden and bubbly.