For the steak:
1) Position a grill pan on the rack closest to the grill and preheat to high. Rub the steak lightly with some olive oil and sprinkle with the herbes de Provence and salt and pepper, to taste. Put the whole tomatoes, spring onions, and garlic cloves in a large bowl, drizzle with about 1 tbsp of the olive oil and sprinkle with salt and pepper to taste. Turn the vegetables until they are lightly coated.
2) Carefully lay the steak in the center of the hot pan, arrange the vegetables around it, and grill until the steak is brown but still tender to the touch and the vegetables are charred, 5 to 6 mins. Turn the steak and the vegetables and grill another 5 to 6 mins, until the steak is medium rare (an instant-read thermometer inserted crosswise into the side of the steak registers 60C) and vegetables are charred. Transfer the steak and vegetables to a cutting board.
For the relish:
1) Core the tomatoes, squeeze the garlic cloves from their skins, and chop them both with the spring onions to make a chunky relish. Transfer the vegetables and all their juices to a bowl and stir in the remaining 4 tbsp or so of the olive oil, the vinegar, and the Worcestershire sauce. Season with salt and pepper, to taste. Slice the meat against the grain and on an angle, transfer to a plate or platter, and serve with the relish and toasted bread.