For the BBQ sauce. Soak the chipotle chillies in a quarter cup of hot water overnight. Drain the liquid and keep to one side. Pat dry the chipotles, then slice.
Warm two tablespoons of oil in a saucepan over low heat. Fry the chipotle for a few minutes until they become brittle, then add the onions, garlic and salt, and cook until soft.
Add the brown sugar and vinegar. Bring to a boil and simmer for 10 minutes. Add the remaining ingredients and the water set aside in step one. Bring to a boil, reduce heat to a simmer and cook until the tomatoes are soft, about 40 minutes. Process with a hand-held blender then push through a fine metal sieve with the back of a spoon. Return sauce to a pot and put on the stove. Cook on a low heat for another 15 to 30 minutes until the flavours combine and sauce becomes glossy.
For the steak, remove the steaks from the fridge two hours before cooking and season liberally with salt.
Preheat the barbecue to hot. Rub steaks with a little extra virgin olive oil.
Place the steaks on the grill and cook, turning half way. For a rare steak, allow about six to seven minutes on each side depending on thickness; add a couple of minutes each side for medium.
Put the steaks on a plate, cover with foil and keep warm. Rest for 10 minutes.
Place the t-bone on each plate and give a good grind of pepper. Put a lemon wedge with each steak and serve the barbecue sauce at the table to share.