Slice the ciabatta bread in half.
Add olive oil to a pan and toast the cut side of the ciabatta lightly.
Season chicken breast with salt and pepper, then cook it in the pan.
Thinly slice lettuce, tomato, and onion.
Layer lettuce, chicken breast, tomato, and onion on the toasted ciabatta.
Drizzle mustard sauce and cover with the bread to finish.