1.Preheat Oven To 400 Degrees F (200 Degrees C). Wash Chicken Breasts, Pat Dry With Paper Towels, And Cut Each Breast In Half. 2.Heat 2 Tablespoons Oil In A Large Skillet Over Medium-High Heat. Sear Chicken Briefly, Turning Until No Longer Pink, About 5 Minutes. 3.Meanwhile, In Another Large Skillet, Heat 2 Tablespoons Oil Over Medium-High Heat. Stir In Mushrooms, Onions, And Garlic, If Using; Cook Until They Are Nice And Soft, About 5 Minutes. 4.Pour Contents Of Both Skillets Into A Baking Dish, Sprinkle Cheese Over The Top, And Bake About 5 Minutes.