1. In a large nonstick skillet, melt 2 tbsps of butter over medium high heat and sauté the mushrooms for 3 to 5 minutes, stirring constantly.
2. Cut each chicken breast in half lengthwise and place them between 2 sheets of heavy-duty plastic wrap and flatten to ⅛ inch thickness.
3. Coat the chicken in the flour and cook for 3 to 4 minutes per side in 1 or 2 tbsps of butter over medium heat in a nonstick large skillet.
4. Remove the chicken from the skillet and leave the drippings in the skillet.
5. Sprinkle the mushrooms over the chicken.
6. In the same skillet, add the broth and boil. Reduce the heat and simmer for 10 minutes stirring occasionally. Season with salt and pepper and pour the sauce over the chicken.
7. In a bowl, mix together the cheeses and onions and sprinkle over the chicken.
8. In a preheated oven to 450°, bake the chicken for 12 to 14 minutes.