To make the mint sauce, mix together the mint, vinegar, sugar and oil. Stir until the sugar dissolves and set aside.
Preheat the oven to gas 6, 200°C, fan 180°C. Rub the lamb with a drop of oil and the paprika. Heat a griddle pan and cook on each side for 3-4 minutes. Toss the onion in the remaining oil and add to the pan. Remove the lamb and leave to rest. Continue to cook the onions until softened and just charred, but not burnt.
Warm the rolls in the oven, then halve. Pour the meat juices over the bread. Slice the lamb and layer with the onions and a spoonful of the sauce.
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