Place chicken, vegetables, bay leaf, and peppercorns in a pan and cover with water.
Slowly bring to a boil and simmer for 30 minutes until the chicken is cooked.
Remove the chicken with a slotted spoon and keep it warm on a plate.
Strain the poaching liquid and reserve 450ml.
Melt butter in a pan, stir in flour to make a roux, and gradually add the reserved liquid.
Add white wine and bring the sauce to a boil.
Whisk egg yolk and cream together and gradually stir the mixture into the sauce.
Return chicken to the sauce, remove from heat, and season with salt, pepper, and lemon juice.
Serve the chicken over mashed potatoes and garnish with chopped parsley.