Season both sides of chicken breast with salt, pepper and garlic powder.
In a large skillet, melt the butter over medium heat. Add mushrooms and onions and sprinkle with 1/4 tsp salt and pepper. Cook until tender, stirring occasionally, about 10 min.
Add chicken, nestle it in the mushroom mixture so it makes contact with the pan. Cook 7-10 minutes on each side, or until cooked through. Continue to stir mushrooms occasionally.
Remove chicken to a platter, sprinkle cheese on chicken and place mushroom mixture on top. Allow to sit a few minutes for cheese to melt. To serve from pan (less dishes)- Once chicken is cooked, sprinkle it with shredded cheese while still in skillet. Turn off heat and let cheese melt.