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 with paper towels.
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 and internal temperature reaches 165°F.
05 - Place a cheese slice on each chicken piece during the last minute of grilling. Cover grill or tent with foil for 1 minute to melt cheese.
06 - Toast sandwich rolls on the grill or in a toaster until lightly browned.
07 - Combine mayonnaise, Dijon mustard, and lemon juice in a small bowl. Mix until smooth.
08 - Spread sauce on both halves of each roll. Layer with lettuce, sliced tomato, cheesy grilled chicken, and red onion. Close sandwich and serve warm.