Preheat oven to 160°C (140°C fan). Place tomatoes, onions, salt and pepper to taste and 1 tbsp of the olive oil in baking dish and toss to combine. Move onions to centre of dish and tomatoes to the outside.

Finely chop rosemary, oregano, garlic, and salt and pepper together on board until it resembles a chunky paste. Place in bowl and mix with 3 tbsp of the olive oil. Drizzle over both sides of lamb and massage to coat well.

Cook lamb Place lamb on top of onions, cover with a layer of foil and roast in oven for 2 hours. Remove foil and continue to cook for a further 3 hours until meltingly tender.

Prepare potatoes Once lamb is in oven, make incisions, about 2mm apart, halfway through each potato. Place on baking tray, drizzle with remaining olive oil, season with salt and pepper to taste and toss to coat. Turn potatoes cut side up and place on lower rack of the oven to roast until lamb is ready to come out (that is, for 5 hours!).

Remove lamb from oven and place on board to rest for at least 15 mins. Drain tomatoes and onions on paper towel. Meanwhile, increase oven temperature to 250°C (230°C fan). Transfer potatoes to top rack of oven for about 15 mins to crisp up.

Serve Pull the meltingly tender meat from the bone using tongs or 2 forks, and serve with the soft, sweet tomatoes and onions and crispy potatoes. Drizzle with cooking juices.

Tip Hasselback potatoes are a Swedish version of roast spuds. Cut into a fancy accordion shape, these seasoned potatoes turn out deliciously crisp on the outside and tender on the inside.