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