01 - Preheat grill or grill pan to medium-high heat.
02 - Slice each chicken breast horizontally to create 4 thin cutlets.
03 - Mix olive oil, garlic powder, smoked paprika, salt, and black pepper in a bowl. Coat chicken cutlets evenly with marinade.
04 - Grill chicken for 4-5 minutes per side until internal temperature reaches 165°F and chicken is fully cooked.
05 - Place cheddar cheese slice on each cutlet during last minute of grilling to melt, if using.
06 - Combine mayonnaise, Dijon mustard, lemon juice, and parsley in small bowl to create creamy spread.
07 - Lightly toast sandwich buns on grill, cut side down, for about 1 minute.
08 - Spread creamy sauce on both bun halves. Layer with lettuce, grilled chicken, tomato slices, and red onion. Top with remaining bun half.
09 - Serve immediately.