Preheat the oven to 200°C.
Slice the chicken lengthways to form a pocket and stuff with 55g of Stilton.
Secure the pocket with cocktail sticks and season with salt and pepper.
Melt butter with oil in a pan and fry chicken for two minutes per side until golden.
Transfer the pan to the oven and bake for 8 to 10 minutes until cooked through.
Boil sweet potato cubes in salted water for ten minutes until soft.
Drain the potatoes and mash with butter and seasoning.
Heat oil and butter in a small pan for the sauce.
Sauté the onion for three minutes, add garlic for one minute, then add wine and reduce.
Add cream and simmer for two minutes before stirring in the remaining Stilton until melted.
Stir in chopped chervil and season to taste.
Plate the mash using a 5cm ring, top with chicken, and spoon sauce over the dish.