Pound chicken breasts to 1/2 inch thickness. Sprinkle both sides of each chicken breast liberally with paprika, salt, pepper, and garlic powder.
In a large skillet, melt the butter over medium heat. Arrange chicken breasts in the pan, cover, and cook for 10 minutes. Turn chicken breasts over, and layer the thinly sliced onions and mushrooms on top of the chicken. Cover, and cook for 10 minutes.
Remove lid, and mix onions and mushrooms into the butter sauce. Reduce heat to low, and cook uncovered for 5 minutes.