01 - Preheat a grill or grill pan to medium-high heat.
02 - Slice each chicken breast in half horizontally to create 4 thin cutlets of even thickness.
03 - Whisk together olive oil, garlic powder, smoked paprika, salt, and black pepper in a bowl. Coat chicken cutlets thoroughly with the marinade.
04 - Place chicken on the preheated grill. Cook for 4 to 5 minutes per side until internal temperature reaches 165°F and meat is no longer pink in the center.
05 - During the final minute of grilling, place a cheddar cheese slice on each cutlet. Close grill lid briefly to melt cheese.
06 - Combine mayonnaise, Dijon mustard, lemon juice, and fresh parsley in a small bowl. Mix until smooth and creamy.
07 - Place sandwich buns cut side down on the grill for about 1 minute until lightly golden and crisp.
08 - Spread creamy sauce on both halves of each toasted bun. Layer with romaine lettuce, grilled chicken with melted cheese, tomato slices, and red onion. Top with remaining bun half.
09 - Serve sandwiches immediately while chicken is warm and cheese is melted.