Preheat oven to 220℃ | 430℉. Combine the sundried tomatoes in their oil with 4 garlic cloves in a bullet blender or food processor. Process until smooth and a paste has formed. Add the salt (it will be salty but is needed to roast the lamb).
Place the lamb into a roasting tray; rub the mixture all over the lamb to evenly coat. Slice about 6 shallow openings (or slits) around the lamb (top and bottom of the roast) and rub the tomato mix into the slits. Slice the remaining clove of garlic into thin slices and place one or two slices into each opening. Sprinkle with a little extra salt (optional).
Cover tray with foil and place onto the middle rack of your oven to bake for 40 minutes (for medium rare - or leave longer until cooked to your liking). Remove tray from oven; uncover; place lamb back in your oven onto the middle rack; change oven settings to grill/broil on high heat and grill for a further 5-10 minutes, or until golden, char grilled crispy on top. Rotate lamb to grill for a further 5-10 minutes to crisp the underside.
Remove from oven and allow to rest for a good 5-10 minutes. Pour the juices from the pan into a serving jug and pour the juices over the lamb to serve.
Make sure to cook meat covered, until the middle is cooked to your liking, before removing and charing outside. Once foil is removed, the outside will cook further and quicker than inside.