01 - Preheat the grill or grill pan to medium-high heat.
02 - Slice chicken breasts in half horizontally to create four thinner pieces. Pat dry.
03 - In a small bowl, mix olive oil, garlic powder, smoked paprika, salt, and black pepper. Rub mixture evenly over the chicken pieces.
04 - Grill chicken for 4–5 minutes per side, or until fully cooked (internal temperature reaches 165°F/74°C).
05 - In the last minute of grilling, place a cheese slice on each chicken piece. Cover grill or tent with foil for 1 minute to melt cheese.
06 - While chicken cooks, toast sandwich rolls on the grill or in a toaster until lightly browned.
07 - In a small bowl, combine mayonnaise, Dijon mustard, and lemon juice for the sauce.
08 - Spread the sauce on both halves of each roll. Layer with lettuce, sliced tomato, cheesy grilled chicken, and red onion. Close the sandwich and serve warm.