Place chicken into a medium-sized pot. Cover with about 2 L chicken stock, bring to a boil over high heat, then lower heat to simmer for 5 minutes.

Remove pot from heat and skim excess foam and fat on top.

Add herbs (if using) and vegetables, except for the sweet peas, to the chicken pot. Season lightly with salt and pepper. Simmer over medium low heat for 20 to 25 minutes. At this point, remove the chicken breasts and set aside to cool. Continue to cook chicken legs for an additional 5 to 10 minutes.

Remove the chicken legs and set aside with the breasts to cool. Strain the vegetables and discard the herbs from the stock and set aside separately.

Discard the skin and bones from the legs and breasts. Pick the meat from the bones, keeping the meat in clean bite-sized pieces, and set aside.

In a medium-sized pot over low heat, melt the butter. Add vegetables to the pot and stir gently. Add flour (while stirring) and continue to cook for 2 minutes over low heat. Little by little, add remaining chicken stock into the flour and vegetable mixture, stirring to incorporate well. Once incorporated, add cream, mustard and truffle oil. 

Add chicken, sweet peas and lemon juice to mixture. Remove from heat and let cool. Cover lightly and place in the fridge to chill.

To assemble, preheat oven to 375 F. Place the finished pot pie filling into an ovenproof low-sided casserole dish.

Working quickly, roll out the chilled puff pastry to approximately 1/2-cm in thickness, and large enough to give you a 2-cm lip for sealing.

Brush egg wash over the edge of the puff pastry and place over the casserole, with the egg wash side down. Press the edges of the pastry well against the casserole to form a seal. Brush top with egg wash.

Bake for 25 to 30 minutes, or until filling is bubbly and top is golden brown.