Heat olive oil in a frying pan over medium heat.
Add the chicken breast and season with salt, onion powder, paprika, cayenne pepper, and black pepper.
Cook for 8–10 minutes, turning halfway through, until fully cooked.
Remove the chicken from the pan and allow it to cool slightly.
Shred the chicken using two forks.
Transfer the shredded chicken to a large mixing bowl.
Add the chopped red onion, pickles, mayonnaise, and mustard.
Mix well until creamy and evenly combined.
Toast or grill the bread slices until lightly golden.
Spread sauce on one slice of bread and top with lettuce and chicken mixture.
Place a second slice of bread on top and spread with more sauce.
Add lettuce, cheese, cucumber, tomato, chicken salami, and additional chicken mixture if desired.
Finish with a third slice of bread.
Repeat with the remaining ingredients.
Cut each sandwich diagonally and serve immediately.