Combine the beef mince, water, sauce, onion and basil. Mix lightly, shape into 4 burgers.
Preheat the barbecue flat-plate or pan to moderately-high. Lightly brush or spray the burgers with oil. Add burgers to the barbecue or pan, after 1-2 minutes lower the heat to moderate.
Cook for 6-7 minutes each side or until thoroughly cooked. Turn them once only.
While burgers are cooking place the mangoes, onion, vinegar, sugar and chilli in a small pan. Cook over a low heat until mangoes are just tender, and relish is syrupy. To serve, place lettuce on rolls, add burgers, and add a spoon of mango relish to each.