Pour juice into a saucepan and add the cumin and sugar, start to heat on a medium heat on the hob.
Once all the sugar has dissolved, remove from the heat and leave to cool down.
Score the fat on the lamb, and pour over the juice mixture, then cover.
Keep in the fridge for at least an hour – or overnight if you have time.
When you’re ready to cook the lamb –Take the lamb out of the fridge and leave to get to room temperature before cooking, heat a heavy bottomed pan with the olive oil, and add the garlic cloves whole.
Add the lamb to the heat – fat side down, and pour in the excess marinade, cover the pan and leave to cook for 10 minutes.
If you're on a barbecue, on a medium to high heat place the meat on the grill and cook for 15 minutes each side - brushing over the remaining marinade.
Then turn over – the fat side should look dark and shiny from the marinade, and cook for a further 10 minutes – with the lid on.
The lamb should be soft and slightly pink inside.