Preheat the oven to 200'C.
Trim any excess fat and sinew from the tenderloin.
Rub the salt and pepper onto the pork with the olive oil.
Heat a frying pan until hot and then sear the pork on all sides. Add the garlic, rosemary and a couple of slices of lemon to the pan and roast the pork for 7 - 10 minutes. Remove from the oven and allow to rest in the frying pan for atleast 5 minutes.
Peel and core the apples, then cut into wedges. Put the butter and sugar into a saucepan and place over a medium heat. Cook until the butter and sugar have formed a light caramel. Add the apple wedges and cook for 5 minutes until the apples have softened and are nicely coated in the caramel.
Cut the shallots in half and peel them. Slice them into thin slices.
Heat a large non stick pan and add the sunflower oil. When the oil is hot add the onions, with the salt and sugar. Cook until dark and golden. When the onions are dark in colour, add the white wine deglaze, add the butter and the pepper.
Remove the pork fillets from the frying pan that they were cooked in. Heat the resting juices over a medium heat and add the chicken stock. Bring to the boil and reduce by half. Stir in the mustard and then taste the sauce. Adjust with salt and pepper if needed.
«Make sure you skim all excess of fat before adding the chicken stock to the sauce otherwise the sauce will be greasy.»