In a large bowl, combine soy sauce, salt, pepper, and lemon juice. Stir.
Put-in the chicken breasts and marinate for at least 3 hours.
Grill chicken until each side is done. Slice into strips, and then set aside.
Heat a pan and then put-in olive oil.
Add the onion and cook in medium heat until caramelized. Set aside.
Combine mayonnaise and Dijon mustard. Mix and spread a reasonable amount of mixture on each side of the sliced sub bread.
Arrange sliced BBQ chicken and caramelized onion in the sub bread.
Wrap or put in a plate.
Serve. Share and enjoy!