Preheat the oven to 190C/375F/Gas 5.
Using a sharp knife, slit the skin of each of the chicken breasts and place three thin slices of truffle under the skin of each breast, retaining the remaining slices of truffle.
Fold back the skin and season both sides with salt and freshly ground black pepper.
Finely slice the remaining truffle and set aside.
Heat an ovenproof frying pan until medium-hot, then place the chicken in the pan, skin-side down, and cook for 2-3 minutes until golden-brown. Make sure the pan is not too hot so that the skin does not shrink back too much.
Turn the chicken over and fry for a further minute, then place in the oven to roast for about 15-20 minutes, or until cooked all the way through (you will know the chicken is cooked as the juices will run clear when a skewer is inserted into the breast).
Meanwhile, heat a saucepan of salted water until boiling. Add the leeks and boil for one minute. Prepare a bowl of ice cold water and, once the leeks have cooked, add them to the bowl of iced water. Once the leeks are cool, drain again.
Repeat this boiling, draining, refreshing and draining process, first with the potato, and then with the pancetta.
Remove the chicken breasts from the oven and place them onto a plate. Cover with aluminium foil and leave in a warm place to rest for about five minutes.
Heat a frying pan until hot and add the olive oil and the refreshed potato and pancetta. Fry for 1-2 minutes until the potato and pancetta are golden-brown.
Add the leeks and most of the set-aside sliced truffles, along with the chervil.
Drain and discard any fat from the frying pan, add a splash of water and stir to make a sauce. Add a knob of butter and the remaining slices of truffle.
Divide the potatoes, pancetta, leeks and truffle between two plates, retaining the liquid for drizzling.
Carve each chicken breast into three and place on top of the vegetables and pancetta. Drizzle the remaining sauce over.
