Clean the soup vegetables, carrot, and onion. Place the whole chicken in a pot with 0.75 liters water, soup vegetables, onion, carrot, bay leaf, chicken broth, and allspice grains. Bring to a boil over medium-high heat, then reduce to medium and simmer for 45-60 minutes until the thickest part of the thigh reaches 74°C / 165°F.
Remove the chicken from the pot and cool until safe to handle. Strain the cooking stock into a clean bowl, discarding the vegetables and solids. Remove the meat from the bones and cut into small pieces.
Melt the butter in a saucepan over medium heat. Add the flour and stir constantly for 1-2 minutes until the mixture turns golden brown, creating a roux.
Gradually add the strained cooking stock to the roux while stirring constantly to create a smooth sauce.
Stir in the capers. Season the sauce to taste with soy or Worcestershire sauce, a pinch of sugar, and lemon juice.
Return the chicken pieces to the sauce and warm through over medium heat for 5 minutes, stirring occasionally.