For the dressing, place the garlic, chilli, cumin seeds and a little salt in a mortar and crush with a pestle to a smooth paste. Heat the olive oil in a pan and, when hot, add the garlic paste. Remove from the heat and leave to stand for 5 minutes, before adding the orange juice, vinegar and tomato ketchup. Season to taste, leave to cool and refrigerate until needed.
Heat a ridged grill