Slice the chicken breast into 1cm thick pieces.
In a bowl, mix the chicken breast with chili powder, salt, pepper, and olive oil.
Grill the chicken breast on both sides until golden brown.
Cut the whole wheat bread in half and lightly toast it, then spread mustard sauce on it.
Layer lettuce, grilled chicken breast, tomato slices, and onion in that order on the bread.
Top with the other bread slice to complete the sandwich.