01 - In a small bowl, combine the olive oil, smoked paprika, ground cumin, garlic powder, onion powder, cayenne pepper if using, dried oregano, salt, and black pepper. Mix thoroughly until a smooth paste forms.
02 - Place the chicken breasts in a resealable plastic bag or shallow dish. Pour the spice marinade over the chicken and toss to coat evenly on all sides. Allow to marinate for at least 15 minutes at room temperature, or refrigerate overnight for deeper flavor penetration.
03 - Preheat a grill or grill pan over medium-high heat. Remove the chicken from the marinade and grill for 6 to 8 minutes per side, until fully cooked through and juices run clear. The internal temperature should reach 165°F. Transfer to a plate and let rest for 5 minutes before slicing or serving whole.
04 - While the chicken rests, stir together the mayonnaise and Dijon mustard in a small bowl until well blended.
05 - Spread the mayo-mustard mixture generously on the bottom half of each toasted bun. Layer with lettuce leaves, grilled chicken, tomato slices, red onion rings, and pickle slices if desired. Cap with the top bun half.
06 - Serve the sandwiches immediately while the chicken is still warm. Pair with potato chips, french fries, or a fresh side salad if desired.