Pound chicken breasts to an even thickness, about 3/4-inch. Transfer to a large resealable zip-tight bag.
4 boneless, skinless chicken breasts
Whisk mayonnaise, honey, dijon mustard, and lemon juice in a small bowl. Pour half of the honey mustard sauce into the bag with the chicken and transfer to the fridge for marinate for at least 30 minutes or overnight. Reserving the remaining sauce for later.
1/2 cup mayonnaise, 1/2 cup honey, 1/2 cup dijon mustard, 2 teaspoons lemon juice
Preheat oven to 400°F.
Cook bacon until crispy over medium-high heat in a large cast iron skillet or other oven-safe pan. Remove bacon from the pan to a paper towel-lined plate to drain.
8 slices bacon
Drain all but about 1 tablespoon of the bacon grease. Add mushrooms and onion to the same pan that was used to cook the bacon. Saute over medium-high heat for 5-7 minutes until softened and lightly browned. Transfer to a plate and set aside.
8 ounces mushrooms, 1 medium yellow onion
Add butter to the same pan. Remove the chicken breasts from the marinade and season on both sides with the salt, pepper, and smoked paprika. Once the butter has melted, add the chicken breasts and cook for 5 minutes without moving until a golden brown crust develops on the bottom of each breast. Flip using tongs and cook for another 3 minutes on the other side. The chicken won't be completely cooked through at this point.
1 teaspoon Kosher salt, 1/2 teaspoon freshly ground black pepper, 2 Tablespoons salted butter, 1/2 teaspoon smoked paprika
Remove the pan from the heat and evenly arrange the bacon slices on top of each piece of chicken. Top with the sauteed mushrooms and onions, then sprinkle with the shredded cheese.
1 cup shredded cheddar cheese, 1 cup shredded Monterey Jack cheese
Transfer the pan to the oven and bake uncovered for 8-10 minutes until the chicken reaches an internal temperature of 165°F when tested with a digital meat thermometer.
Garnish with the chopped parsley and serve with the reserved honey mustard sauce on the side.
2 Tablespoons chopped fresh parsley