01 - Preheat grill or grill pan to medium-high heat.
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 pepper in a small bowl. Rub mixture evenly over all chicken pieces.
04 - Place chicken on hot grill and cook 4-5 minutes per side until fully cooked, reaching internal temperature of 165°F.
05 - During the last minute of cooking, place a cheese slice on each chicken piece. Cover grill or tent with foil for 1 minute until cheese melts.
06 - Toast sandwich rolls on the grill or in a toaster until lightly golden and crisp.
07 - Whisk together mayonnaise, Dijon mustard, and lemon juice in a small bowl until smooth and well combined.
08 - Spread sauce on both cut sides of each toasted roll. Layer with lettuce, tomato slices, cheesy chicken, and red onion. Close sandwich and serve immediately.