Preheat grill or barbecue to medium–hot. To grill tomatoes, place cut–side up on a baking tray. Sprinkle with garlic. Place under grill 2 to 3 minutes. Turn and cook another 2 minutes. To barbecue, place tomatoes cut–side down on a piece of cooking foil. Cook over fire 2 to 3 minutes. Turn; cook another 2 minutes. Tomatoes should still hold their shape.
To make croutons, slice bread into small cubes. Heat 3 tablespoons of the olive oil in a nonstick frying pan over medium heat. Add bread cubes; cook until crisp on all sides. Set aside.
Tear rocket into small pieces. Arrange on serving plates. To make vinaigrette, whisk balsamic vinegar, lemon juice, salt, pepper and remaining oil until combined.
Place grilled tomatoes on top of rocket. Drizzle with vinaigrette. Add olives and croutons.