Preheat the oven to 200°C.
Flatten the chicken mini fillets between cling film with a rolling pin to 5mm thickness.
Butterfly the main chicken breasts by cutting 1.5cm deep and opening the sides.
Purée the chicken thigh meat in a food processor.
Blend in the egg white and then the double cream until combined.
Season the mousse with salt, pepper, and chervil.
Fill each breast with mousse, fold over, and seal with a flattened mini fillet.
Sear chicken in olive oil for 2 minutes per side and roast for 20-25 minutes.
Sauté shallots in half the butter for 3 minutes.
Cook the morel mushrooms for 2 minutes.
Add sherry and boil to reduce the liquid by half.
Add chicken stock and simmer until reduced by half.
Add cream and simmer until reduced by a third.
Stir in parsley and seasoning to the sauce.
Cook the pasta in boiling salted water and drain.
Toss pasta with remaining butter, olive oil, chervil, and seasoning.
Slice the rested chicken and serve with sauce and pasta.