Place the lamb in a shallow non-metallic dish. Place the spring onions in a food processor with the ginger, sesame oil, soy, sherry, sugar, sesame seeds and peppercorns. Blend to a thick paste and then rub all over the meat in a shallow non-metallic dish (or use a turkey bag if you have one), then cover with clingfilm and chill for up to 2 days or leave to stand at room temperature for 2-3 hours if time is short.
Preheat the oven to 230°C (475°F), Gas mark 9. If the lamb has been chilled in the fridge, bring it back to room temperature, then place, cut side up on a rack in a large roasting tin and season with salt. Roast in for 20 minutes, then turn over and roast for another 15 minutes for rare.
Meanwhile, make the salsa. Bring a medium-sized pan of water to the boil, then add the garlic and blanch for 2 minutes. Drain and repeat the blanching process again. Drain well on kitchen paper and then peel off the skins; they should come off easily.
Wipe out the pan, add the olive oil and then place over a medium-high heat. Add the garlic cloves and cook for 4-5 minutes until golden on all sides, stirring occasionally. Add one tablespoon of the sugar, the balsamic, a quarter teaspoon of salt and 6 tablespoons of water. Bring to the boil, then reduce the heat and simmer on a medium heat for 3-5 minutes until most of the liquid has evaporated and the garlic cloves are covered with a nice thick syrup. Leave to cool.
Pare the rind from the orange, avoiding the white pith and then cut into 1mm julienne and put into a small pan. Squeeze out the juice from the orange and make up to 100ml (3 ½fl oz) with water. Pour over the julienne and add the remaining teaspoon of sugar. Bring to a simmer over a medium heat and cook for 6-8 minutes until the liquid has reduced by about a third.
To finish the salsa, stir the chilli, mint and lemon juice into the caramelized garlic syrup and then stir in the orange julienne without the syrup.
Remove the lamb from the oven and leave to rest in a warm place for 10 minutes. If you don’t like your lamb too pink you can cover it with foil at this point and it will continue to cook. Carve into slices and add any of the cooking juices to the orange salsa. Taste and add a little of the remaining orange syrup if you think it needs it. Arrange the carved lamb on warmed plates with some of the caramelised garlic and orange salsa on the side or on top. Add some of the charred little Gem lettuce wedges or cabbage to serve if liked.