Place chicken breasts on a cutting board, cover with plastic wrap, and pound to an even thickness of about 3/4 inch (2cm) using a meat mallet or rolling pin.
In a mixing bowl, whisk together olive oil, fresh lemon juice, lemon zest, cracked black pepper, kosher salt, minced garlic, onion powder, Italian seasoning, and smoked paprika until emulsified.
Place the pounded chicken in a zip-lock bag or shallow dish, pour the marinade over it, massage to coat well, and refrigerate for at least 30 minutes or up to 4 hours.
Preheat your outdoor grill or indoor grill pan over medium-high heat for 10 minutes, then clean and lightly oil the grates.
Remove chicken from marinade, let excess drip off, and place on the hot grill. Cook undisturbed for 5-7 minutes on the first side until seared, then flip and cook 5-7 more minutes until internal temperature reaches 165°F (74°C).
Transfer chicken to a cutting board, tent with foil, and rest for 5 minutes. Garnish with fresh parsley and serve with lemon slices.