Oven Preparation: Preheat the oven to 350°F (177°C). Prepare a 9x13-inch baking dish by lightly greasing or lining with parchment paper.
Seasoning and Preparation of Chicken: Season both sides of the chicken breasts liberally with kosher salt and freshly ground black pepper. Dredge each piece in all-purpose flour, ensuring a light, even coating, and shake off any excess flour.
Browning the Chicken: Heat olive oil in a large skillet over medium heat. Once the oil is hot, add the chicken and brown on both sides. The chicken need not be cooked through at this stage as it will continue cooking in the oven.
Sautéing the Mushrooms: In the same skillet, increase heat to medium-high and melt the butter. Add the sliced mushrooms and season with salt and pepper. Sauté until the mushrooms are golden brown. Deglaze the skillet by adding sherry wine, allow it to cook for 1-2 minutes.
Preparing the Sauce: Stir the cream of mushroom soup into the skillet with the mushrooms and sherry, mixing thoroughly. Heat the mixture until it is uniformly warmed.
Assembling the Casserole: Place the browned chicken pieces in the prepared baking dish. Pour the mushroom and soup sauce evenly over the chicken. Arrange slices of cheese atop the sauce-covered chicken.
Baking: Cover the baking dish with aluminum foil and place in the preheated oven. Bake for 30 minutes. After baking, remove the foil and switch the oven to broil. Broil the casserole for 1-2 minutes or until the cheese is bubbly and lightly browned.
Final Steps: Remove the casserole from the oven and allow it to rest for five minutes. Garnish with chopped fresh parsley before serving to enhance the dish’s visual appeal and add a fresh flavor contrast.