Spray pan with nonstick spray, warm on medium.

Sprinkle both sides of chicken with oregano, salt, garlic powder, and pepper.

Cook chicken in skillet 8-10 minutes, turning once, until juice is no longer pink when center of thickest piece is cut. Remove from heat.

Spray skillet and saute onions and mushrooms for 5 minutes.

In skillet mix chicken and veggies. Sprinkle cheese and remove from heat. Let stand until cheese is melted.