Prep everything first. Slice the mushrooms, mince the shallot and garlic, cut the butter into pieces, and measure the Marsala wine and chicken stock. Once cooking starts, this recipe moves quickly, so having everything ready makes it much easier.
Prepare the chicken cutlets. Butterfly two thick chicken breasts or pound chicken to about ¼-inch thickness. Season both sides generously with salt and pepper.
Make the egg batter and dredging station. In a shallow bowl, whisk together the eggs, cream or milk, Parmesan cheese, Italian seasoning, salt, and pepper until smooth and slightly frothy. Place the gluten-free flour in a separate shallow dish.
Heat the oil. Heat olive oil in a large skillet over medium-high heat until hot and shimmering. A small piece of gluten-free bread should sizzle immediately when added.
Coat and cook the chicken. Dip each cutlet in flour, then into the egg batter, and place it into the pan. For a thicker coating, dredge the chicken in flour again after the egg. Cook two cutlets at a time for 4–5 minutes per side, until golden and cooked through (internal temperature should reach 165°F). Transfer to a plate.
Wipe out the pan if needed. If there is excess oil or dark browned batter bits in the skillet, carefully discard the oil and wipe the pan clean. This helps prevent bitterness and keeps the sauce flavorful.
Cook the mushrooms. Reduce the heat to medium and add 2 tablespoons of butter to the skillet. Once melted, add the mushrooms and cook for 5–7 minutes, stirring occasionally, until softened and lightly browned. Season with salt and pepper once the mushrooms cook down. If they release a lot of liquid, carefully drain off some of it before continuing.
Add the aromatics. Push the mushrooms to one side of the skillet and drizzle a little more olive oil into the empty space. Add the shallots and garlic and cook for 30–60 seconds, just until fragrant. Do not let the garlic brown. Stir the shallots and garlic into the mushrooms.
Deglaze with Marsala. Add the Marsala wine and simmer for about 3 minutes, allowing it to reduce slightly and cook off the alcohol.
Build and reduce the sauce. Add the remaining butter and chicken stock. Bring to a gentle boil, then reduce the heat and simmer for 6–8 minutes (up to 10 minutes), until the sauce reduces by about half.
Finish the dish. Season the sauce with salt and cracked black pepper to taste. Return the chicken to the pan for 3–4 minutes, just until heated through and coated in the sauce. Transfer the chicken to a plate or platter and spoon the mushrooms and sauce over the top.