Preheat the oven to 200C/400F/Gas 6.
Place the chicken breasts, skin side-down onto a chopping board. Remove the mini fillets from the main breast. Place the mini fillets between two sheets of cling film on a flat work surface and bash with a rolling pin to flatten to 5mm thickness.
Using a sharp knife, cut into the main breasts from the fattest part down towards the bottom, just 1.5cm/½in deep, not cutting all the way through. Return the knife to the top of the cut and cut to the side, opening out the chicken breast. Repeat with the other side, then open the folds of chicken to open the breast out.
Place the chicken thigh meat into a food processor and blend to a purée.
Add the egg white and blend again to combine, then add the double cream and pulse until combined.
Season with salt and freshly ground black pepper, add the chervil and mix well.
Place a couple of spoonfuls of the chicken mousse into the centre of each opened up chicken breast. Fold the sides of the chicken breasts back over the filling, then place a flattened mini fillet over the top of each, taking care that all the filling is covered and the mini fillet sticks to the chicken breast.
Heat an ovenproof frying pan until hot, add the olive oil and the stuffed chicken breast, skin-side down. Cook for 1-2 minutes on each side, until golden-brown all over, then transfer to the oven to roast for 20-25 minutes, or until completely cooked through. Test the chicken breast is cooked by inserting a skewer into the thickest part of the meat. If the juices run clear, the chicken is cooked. Remove from the oven and leave to rest for five minutes.
For the sauce and pasta, heat a clean frying pan until hot, add half the butter and all of the shallots and cook for 2-3 minutes, until the shallots are just softened.
Add the morels and cook for two minutes.
Add the sherry and boil to reduce the liquid by half. CAUTION: the sherry may catch light, make sure there are no flammable objects nearby.
Add the chicken stock and bring to the boil, then reduce the heat and simmer until the volume has reduced by half.
Add the cream and continue to simmer until the liquid is reduced by a third.
Add the parsley and season, to taste, with salt and freshly ground black pepper.
Meanwhile, bring a pan of salted water to the boil, then add the pasta and cook according to packet instructions. Drain the pasta, then return it to the pan it was cooked in.
Add the remaining butter and all of the olive oil and chervil. Mix well, seasoning, to taste, with salt and freshly ground black pepper.
To serve, slice each chicken breast into three pieces and place onto four plates. Spoon the mushroom sauce over the chicken then spoon some pasta alongside.
