Whisk together the Dijon mustard, honey, mayonnaise, garlic, and onion powder in a medium-sized mixing bowl.
Place the chicken in a gallon-sized ziploc bag. Pour in 2/3 of the marinade. Seal the bag and marinate the chicken for 20 minutes. Save and refrigerate the remaining sauce for later.
While the chicken is marinating, prepare the bacon and cook until crisp. You can use precooked bacon for convenience. Just warm it up according to package instructions.
To prepare the mushrooms, place 2 tablespoons of butter into the cast iron skillet. Make sure it is oven safe. Melt the butter over medium-high heat. Add the sliced mushrooms and cook for 7-10 minutes, or until they are golden brown and tender. Remove the mushrooms from the skillet and set them aside.
Preheat oven to 375°F. Add the olive oil to the same skillet and heat over medium-high heat. Remove the chicken from the marinade leaving behind any excess marinade drippings in the bag. Discard the bag of marinade.
Cook for 4-5 minutes on both sides. There is no need to cook through the chicken, you just need to sear it.
Divide the sautéed mushrooms evenly over each chicken breast. Top with one slice of bacon that has been cut in half to fit on each breast. Sprinkle on 1/4 cup shredded cheese.
Cover the skillet with foil, and place in preheated oven. Bake until the chicken reaches 165°F. This will take about 10-15 minutes. You can remove the foil the last 3 minutes to melt the cheese.
Garnish with fresh parsley, and serve with mashed potatoes and steamed broccoli.