Heat a cast iron skillet on medium high and rub a bit of the butter on the skillet to coat the bottom. Sprinkle the chicken breasts with the seasoning. Place the seasoned chicken in the hot pan, browning them lightly - turn just once.
Remove chicken from skillet and place on a baking sheet.
Put the chicken in the preheated oven and cook for about 20 minutes.
Meanwhile, add the rest of the butter to the skillet. When it is sizzling, toss the mushrooms and onions in. Cook the mushrooms and onions until the mushrooms are softened and the onions are almost translucent. About 10 minutes or so.
Carefully remove the pan with the chicken from the oven and spoon the mushroom and onion sauce over the chicken breasts. Return the chicken to the oven and continue cooking until the chicken reaches an internal temperature of 170 to 180."