Sit the chicken on a rack set over a roasting tray and roast for 15-20 minutes until golden brown.
Transfer the chicken to a large heavy based pan, reserving the fat caught in the roasting tray. Scatter the diced vegetables over and pour in enough cold water to just cover. Add the bouquet garni in and season. Bring to the boil and simmer for 40-45 minutes until tender.
Remove the chicken from the stock and leave to rest and cool slightly. Remove all the meat from the carcass and shred the chicken into small pieces.
Meanwhile, strain the stock through a fine sieve, discarding the vegetables. Return the passed stock to a clean pan. Mix the minced chicken and egg whites together to combine and whisk the mixture quickly into the stock. Very slowly bring the stock to a simmer. Avoid stirring, but whisk once as the stock is coming up to temperature. The chicken and egg white mixture will solidify and rise to the top of the stock. As the stock simmers any impurities will rise to the top and thus clarify the stock. Pass the stock through a fine sieve once again.
Bring a pan of salted water to the boil and blanch the leek strips for 1-2 minutes until softened. Drain and refresh in ice cold water. Drain again.
To serve, arrange some chicken shreds in the center of each serving bowl. Sit the leeks and prunes on top. Ladel the warm stock over (which will warm the other ingredients through). Drizzle some of the reserved chicken fat over and serve at once.