01 - Preheat grill or grill pan to medium-high heat, approximately 375-400°F.
02 - Slice chicken breasts horizontally to create four thinner cutlets. Pat dry with paper towels.
03 - Combine olive oil, garlic powder, smoked paprika, salt, and black pepper in a small bowl. Rub mixture evenly over all chicken pieces.
04 - Place chicken on preheated grill. Cook 4-5 minutes per side until internal temperature reaches 165°F and chicken is no longer pink in center.
05 - Place a cheese slice on each chicken piece during the last minute of cooking. Cover grill or tent with foil for 1 minute until cheese is melted and bubbly.
06 - Toast sandwich rolls on grill or in toaster until lightly golden brown and crisp, about 1-2 minutes.
07 - Whisk together mayonnaise, Dijon mustard, and lemon juice in a small bowl until smooth and well combined.
08 - Spread sauce evenly on both halves of each toasted roll. Layer bottom half with lettuce, tomato slices, cheesy chicken, and red onion. Top with remaining roll half and serve immediately.