Cut the chicken breast into bite-sized pieces.
Mix all the sauce ingredients (soy sauce, gochujang, honey, garlic, pepper, etc.) together.
Marinate the chicken breast in the sauce for 10 minutes.
Heat olive oil in a pan and grill the chicken breast until both sides are golden brown.
Slice the bread (white or whole wheat) in half.
Top the bread with seasoned chicken breast, lettuce, tomato slices, and onion.
Add mayonnaise or mustard sauce to taste.
Cover with another slice of bread to complete the sandwich.