Slice the baguette in half lengthwise.
Wash the vegetables (lettuce, tomato, cucumber) thoroughly and cut them into bite-sized pieces.
Season the chicken breast with salt and pepper, grill in a pan, let it cool, and slice thinly.
Lightly spread olive oil on the inside of the baguette.
Layer the vegetables, chicken breast, avocado, and onion in order on the baguette.
Top with mustard sauce as desired, then close the sandwich.