Preheat the Oven – Set the oven rack to the lower-middle position. Preheat to 375ºF (191ºC).
Prepare the Chicken – Butterfly the chicken breast in half lengthwise, leaving about a ½-inch uncut area on the other side. Repeat with the remaining pieces.
Coat Chicken – In a medium bowl, combine flour, 1 teaspoon salt, and ½ teaspoon pepper. Lightly season each side of the chicken breast with salt and pepper. Dredge the pieces in the seasoned flour, shaking off any excess. Transfer to a plate and set aside.
Make the Filling – In a medium bowl, combine the breadcrumbs, mozzarella cheese, parmesan cheese, sundried tomatoes, parsley, and melted butter.
Stuff the Chicken – Evenly spread the breadcrumb mixture into the pocket of each chicken breast, about 2 to 4 tablespoons, do not overfill. Close the opening with toothpicks to secure the filling inside.
Sear the Chicken – Heat a large oven-proof skillet or Dutch oven over medium heat. Once the pan is hot, add the olive oil. Add the chicken, presentation-side down. Lightly press the meat to make contact with the pan. Cook until golden brown on the surface, about 5 to 7 minutes. Flip and cook for 5 minutes. Transfer the chicken to a plate and set aside.
Saute the Mushrooms – In the same pan used to sear the chicken, melt the butter over medium heat. Add the onions and garlic, and sauté for 1 minute, stirring to ensure the garlic does not brown. Add the mushrooms, ½ teaspoon salt, and ¼ teaspoon pepper. Sauté until tender, about 5 minutes.
Make the Sauce – Turn the heat up to medium-high and add the Marsala wine. Cook until reduced by half, about 2 minutes. Add the chicken stock. Cook until the sauce is reduced by ½ (about ½ cup), about 5 to 7 minutes. Reduce the heat to medium, and stir in the cream. Simmer until the sauce has thickened, about 3 to 5 minutes. Season to taste with salt and pepper.
Bake the Chicken – Transfer the chicken to the pan with the sauce. Add some of the sauce on top. Transfer the pan to the oven. Bake until the meat is no longer pink and the internal temperature reaches 160 to 165ºF (71 to 74ºC), about 10 to 15 minutes, depending on the thickness of the meat. If using smaller chicken breasts, check for doneness, as baking may not be needed.
To Serve – Serve the stuffed chicken breast with the sauce on top and chopped parsley. It pairs well with pasta or mashed potatoes.
Notes
Make Ahead: The chicken can be stuffed 1 day in advance. Store in an airtight container and refrigerate.
Storing: Cool completely and store in anairtight containerfor up to 5 days.
Reheating: Cover and reheat in the microwave on high heat in 15 to 30-second increments until hot.
Make it Gluten-Free: Omit the flour coating, or use cornstarch or cassava flour.