Place chicken, chopped onion, carrot, celery, peppercorns, and bay leaf in a pan and cover with water.
Boil slowly and simmer for 25 to 30 minutes until the chicken is cooked.
Remove chicken to keep warm and reserve 450ml of the strained cooking liquid.
Melt half the butter in a pan, stir in flour to make a paste, and gradually whisk in the reserved liquid.
Add the blanched broccoli and drained water chestnuts to the sauce.
Mix egg yolk with cream and slowly stir into the sauce with a squeeze of lemon juice.
Dice the cooked chicken into bite-sized pieces and stir them back into the sauce.
Fry the sliced onion in the remaining butter until golden-brown.
Top the casserole with fried onions and parsley and serve with baby carrots.