Mix the brown sugar and lime juice together and pour over the steaks, turning to thoroughly coat. Set aside for 10 minutes.
Meanwhile, combine all the salsa ingredients in a bowl and season lightly.
Remove the steaks from the marinade, season, then lay them on a hot barbecue and cook undisturbed for 2-21⁄2 minutes on each side for a medium-rare steak. Add another minute for a medium to well-done steak.
If using a griddle pan, set it over a medium to high heat until smoking hot and cook the steaks as above.
Loosely cover the steaks with foil and leave to rest for 3 minutes before serving with the mango salsa.
If you're barbecuing for lots of people, cook the meat in the oven first then finish it on the barbecue for flavour and to ensure that it is thoroughly cooked. This is particularly important for burgers, sausages, pork and poultry.