Grate the onion into a bowl until you have 3 tbsp grated onion. Leave the remainder in one piece and set aside. Add 1 tsp of the oil, the lime zest, tamarind, cumin, 2 tbsp coriander, smoked paprika and 1 tsp water. Mix well, then use half the spice paste to coat the steaks all over. Mix the rest with the lime juice, remaining coriander and the beans in a serving bowl.
Light the barbecue (or see tip, below, to cook in the oven). Take a large piece of foil and drizzle with the remaining oil. Finely chop the rest of the onion and mix with the sweet potato, pepper and courgette. Wrap in the foil along with the bay leaf, if using, to make a parcel. Cook on the rack over the coals for about 20 mins, turning several times with tongs until the veg is tender. Tip the contents into the beans and toss well. Barbecue the steaks on both side until cooked to your liking, then serve with the warm salad.