Prepare the Chicken: In a shallow dish, combine 1 cup of all-purpose flour, kosher salt, and white pepper. Coat each chicken breast in the seasoned flour, ensuring to shake off any surplus before cooking.

Sear the Chicken: In a large skillet, heat 4 tablespoons of butter and 2 tablespoons of olive oil over medium heat. Once hot, add the chicken breasts and sear for 4-5 minutes on each side, until golden brown. The chicken does not need to be fully cooked through at this point.

Sauté the Vegetables: In the same skillet, add the remaining 4 tablespoons of butter and 2 tablespoons of olive oil. Sauté the sliced mushrooms, onions, and garlic over medium heat for 5-7 minutes, or until the mushrooms are browned and the onions are soft.

Deglaze with Sherry: Pour in the dry sherry wine to deglaze the skillet, scraping up any browned bits from the bottom. Let the wine simmer for about 2 minutes, allowing the alcohol to cook off.

Make the Creamy Sauce: Sprinkle 3 tablespoons of flour over the sautéed vegetables and stir until fully incorporated. Slowly add the chicken stock while continuously stirring to avoid any clumping. Once the stock is fully mixed, add the heavy cream and continue stirring until the sauce thickens, about 5 minutes.

Assemble the Dish: Preheat your oven to 375°F (190°C). In an oven-safe dish, arrange the seared chicken breasts in a single layer. Pour the creamy mushroom and onion sauce over the chicken, making sure each breast is fully coated. Layer the Muenster cheese slices on top of the chicken and sauce.

Bake: Place the dish in the preheated oven and bake for 20-25 minutes, or until the cheese is melted, bubbly, and slightly golden, and the chicken is fully cooked (internal temperature should reach 165°F).

Serve: Remove the Chicken Gloria from the oven and let it rest for 5 minutes before serving. Garnish with fresh herbs if desired and enjoy with rice, pasta, or your favorite vegetable side.