Slice each chicken breast lengthwise through the center to create two thin, even cutlets.
Lightly pound them with a meat mallet if needed so they cook evenly and stay tender.
Whisk the olive oil, lemon juice, garlic, onion powder, oregano, paprika, salt, and pepper in a bowl or resealable bag.
Add the chicken cutlets and marinate in the refrigerator for 30 minutes to 2 hours (avoid marinating too long in heavy citrus, which can make the meat mushy).
Preheat your gas or charcoal grill (or grill pan) to medium-high heat (about 375°F to 400°F) and oil the grates.
Grill the thin cutlets for about 3 minutes per side until well-marked and cooked through (reaching an internal temperature of 165°F / 74°C).
Remove the chicken from the grill and let it rest on a cutting board for 5 minutes.
Transfer the cutlets to a container and chill them completely in the refrigerator before slicing or layering onto cold bread.
