SEASON & DREDGE: Combine the garlic powder, salt, onion powder, and ground mustard in a small bowl. Season the chicken on both sides.
DREDGE: Whisk the flour with ½ teaspoon kosher salt in a shallow baking dish. Combine the grated parmesan and the panko breadcrumbs in another shallow dish. Take each chicken cutlet, dredge it through the flour, dust off excess, dip in the egg mixture, and finally dip it in the parmesan coating. Gently press down so the parmesan and panko adhere. Place the cutlet on a clean plate, and repeat with the remaining cutlets.
COOK: Heat the oil in a 10-inch skillet over medium heat. When hot, add two pieces of the chicken cutlets to the pan and cook for 6-9 minutes, flipping halfway until the chicken is cooked through. Remove the chicken to a wire rack set on a baking sheet. Cook the remaining chicken the same way, adding more oil as needed.
SAUCE: Wipe down the skillet to remove any residue (or you can also do this in another pan.) Add the lemon juice, chicken stock, garlic, and red pepper flakes to the pan. Kick the heat up to high to gain a simmer, then reduce the sauce for 2 minutes. Lower the heat to medium-low, add in 1 tablespoon of butter, swirl the pan or whisk to help the butter melt into the sauce. Repeat this method with the remaining butter, adding 1 tablespoon at a time. Take your time, don’t rush this! When the butter is completely incorporated, stir in the parsley and serve warm drizzled over the cutlets.