Place chicken breasts into a large pan. Add the vegetables, bay leaf and peppercorns and cover with water.
Place the pan over a low heat and slowly bring to the boil, then simmer for 30 minutes, or until the chicken is completely cooked through.
Remove the chicken with a slotted spoon, place onto a plate and keep warm.
Strain the cooking liquid into a bowl, reserving 450ml/Â¾ pint.
Melt the butter in a clean pan over a medium heat. Add the flour and stir well to make a roux. Slowly add the reserved chicken stock to the roux, stirring constantly, to make a thick sauce.
Add the white wine and bring to the boil.
Place the egg yolk and the cream into a clean bowl and lightly whisk together. Gradually add the egg mixture to the sauce, stirring well.
Add the chicken breasts to the sauce. Remove from the heat, season, to taste, with salt and freshly ground black pepper and add a squeeze of lemon juice.
To serve, spoon the mashed potato on serving plates, top with a chicken breast, pour over some sauce and garnish with chopped parsley.