To poach the chicken, place chicken breast into a saucepan with lemon, dill stalks and peppercorns. Add enough water to full submerge the chicken. Place over a high heat. Once water is boiling, turn off the stove, place a lid on the pot and let chicken sit for 45 minutes.
Once chicken is cooked, remove from pot and cut into small one centimetre cubes. Place into the fridge to chill.
In a large bowl combine diced chicken with mayonnaise, dill, parsley, Dijon mustard, salt and pepper.
For the pickled onions, whisk together water, vinegar, sugar and salt and pour over sliced red onion. Cover and place in the fridge for at least one hour.
To assemble sandwich, lay down slices of cucumber on one slice of toasted brioche. Next, scoop on half the herb chicken salad mix. Top with pickled onions and shredded lettuce, followed by toasted brioche. Serve immediately.