Bring chicken broth, peppercorns, and onion to a boil in a large pot.
Season the chicken with salt and pepper and poach in the stock of 8 to 10 minutes depending on the size of the breasts.
In a separate pan lightly brown the leeks and lettuce with olive oil.
Add white wine and reduce to almost dry.
Add thyme sprig, 1 teaspoon of whole butter, salt and pepper and enough of the poaching liquid from the chicken to cover the leeks and lettuce half way.
Reduce the heat to a slow simmer and pan braise until tender.
Remove the chicken and allow to rest for 5 minutes.
Remove the breasts from the bone and sauté skin side down in a non stick pan with olive oil.
Turn over a repeat the process until the chicken is cooked through.
Remove the chicken and heat the sauce in the same pan.
Add cream and chervil and season with salt and pepper.
Place the lettuce in the middle of the plate.
Slice the chicken breasts in three pieces and lay on top of the lettuce.
Arrange the leeks on either side of the chicken.
Sauce the top of the chicken.