. Prepare your barbecue so that it's at the right temperature to cook, but not burn the meat. When the coals are white, they are hot enough to use.
Gently rub the steaks with a touch of vegetable oil, then season with salt and pepper and chargrill for at least 15 minutes, turning them over and moving them around the grill as needed, until evenly coloured but not burnt.
Move the steaks to one side, away from the fiercest heat, so that they do not scorch. Using a skewer or very thin-bladed knife, pierce the steaks and touch the blade to your lips - if it's hot the steaks are done. Transfer them to a plate, and leave them to rest for 15 minutes, covered loosely with foil.
Meanwhile, make the tomato salad. Mix the mustard with the olive oil and vinegar. Then, using a microplane, slice the garlic very finely. Add it to the mustard, oil and vinegar and stir to combine to a dressing. Mix the tomatoes with the red onion in a bowl, season with salt and pepper, then pour the dressing over the top. Set aside to marinate, until you're ready to serve.
Once the steaks have rested it's time to carve them Remove the meat from the bone (if necessary), and slice against the grain to give about 2cm-thick slices. Divide the tomato salad equally between serving plates and scatter over the basil leaves. Place the sliced steak on top of the tomatoes and garnish with some watercress and rocket. Alternatively, serve the sliced steak and the salad on a large serving platter and allow everyone to help themselves.