Melt the butter in a small pan and fry the onion for about 5 minutes until soft.
In a separate bowl mix the breadcrumbs, lemon zest, herbs and a sprinkling of salt and pepper and add the onion and the egg and mix well.
Split the pork fillet ALMOST in half, lengthways, and open out. Spread the stuffing down the length and close.
Stretch the rashers out and wrap round the fillet, overlapping each other slightly. Tie the fillet together with the string. Put into a roasting dish and drizzle with oil.
Roast for 45 minutes.
Take out the pork fillet and baste it with the oil and juices in the pan and place back into the oven for 25 minutes, or until pork measures 97C when measured with an instant read thermometer.
Transfer the pork on to a plate and allow it to rest for 5 minutes before serving.
Cut off the string, cut into portions and serve.
Serve with mash/roast potatoes and fresh veg.
Make a gravy from the juices - put the roasting tin on the hob (mix in 2 teaspoons plain flour) and gradually add stock (3/4 pint chicken or veg) and keep stirring until it is thick and smooth. Simmer for 5 minutes.
Use packet mix sage and onion stuffing in the middle of the pork fillet.