Slice each chicken breast horizontally to create two thinner cutlets (you will have four cutlets). Place each cutlet between plastic wrap or in a zip-top bag and gently pound with a meat mallet or rolling pin until uniform and about ½ inch thick.
Zest both lemons with a fine grater or microplane, avoiding the bitter white pith; set the zest aside. If you want caramelized lemon slices, thinly slice the zested lemons and set the slices aside.
Pat the cutlets dry with paper towels. Press the lemon zest onto both sides of each cutlet so the zest adheres. If the zest does not stick easily, squeeze a little lemon juice from one of the lemons onto the cutlets and then press the zest on.
In a shallow dish, whisk together the flour (3 tablespoons), salt (½ to ¾ teaspoon), cracked black pepper (2½ teaspoons), and garlic powder (½ teaspoon) until evenly combined.
Dredge each lemon-zested cutlet in the flour mixture, coating both sides. Shake off any excess flour.
Heat 1 tablespoon olive oil in a large skillet over medium-high heat until the oil shimmers. Add the cutlets (work in batches if they crowd the pan) and cook about 4 minutes per side, until golden and cooked through (about 165°F internal temperature) — adjust time for thickness as needed.
Transfer the cooked cutlets to a plate and let them rest 2–3 minutes. If using lemon slices, add them to the hot skillet after removing the chicken and cook briefly, about 1 minute per side, until lightly caramelized.
Garnish the chicken with fresh parsley and serve immediately, with the caramelized lemon slices if you prepared them.