Finely chop the onion and 2 sage leaves.
Mix the breadcrumbs with the sage, parsley and plenty of salt and pepper.
Melt the spread in a pan and add onion and garlic. Cook for 5 minutes or until the onion is soft. Add the crumb mixture and combine.
Preheat the oven to 200°C.
Cut a deep slit along the top of the roast (the fat side up) and stuff with most of the mixture. Tie it at intervals with a string. Rub the rest of the mixture under the joint and over the two sides. Put the sage leaf on top of the fat (secured under the string) and season with salt and pepper.
Place the joint in a baking dish and roast for 20 minutes, then turn down the temperature to 180C and bake the loin for a further 20 min/450g.
Let it rest for 10 minutes (under cover) before carving.