For mint stuffing, heat olive oil in a small frying pan over low heat, add onion and a pinch of salt and cook, stirring often over low heat until caramelised, about 10 minutes. Set aside to cool. Meanwhile, place mint leaves, garlic and a pinch of sea salt in a mortar and, using a pestle, coarsely crush, add hemp seeds and parmesan and pound to a paste, stir through oil and season to taste. Combine pesto, breadcrumbs and caramelised onion.
Place lamb skin-side down on a clean work surface and evenly spread stuffing over meat. Starting at the longest sides roll up, tucking in sides and secure with kitchen twine.
Preheat oven to 200°C true fan. Place lamb into a roasting pan, drizzle with olive oil and season with salt and freshly ground black pepper. Roast lamb for 30 minutes, then reduce heat to 160°C and roast for a further 40 minutes or until the internal temperature reads 62°C on a thermometer, for medium-rare. Allow lamb to rest for 10-15 minutes in a warm place before carving.
To serve, remove twine and cut into 3cm thick slices. Serve drizzled with pan juices.