Method
If you’re using a barbecue, light the charcoal well before you want to start cooking, so that it has time to come up to temperature (see our handy guide to getting the most from a charcoal barbecue to learn more). While you wait, make the tomato topping; chop the tomatoes into small dice, removing the cores if they seem tough to you, then tip the tomato pieces into a large bowl. Trim and peel the red onions, then dice them the same size as the tomatoes and add to the bowl. Crush in the garlic clove, then roughly chop the basil or marjoram leaves and add them, too. Measure in the oil and vinegar and give everything a good stir, then season to your taste. Set aside so the flavours can mingle while you prep the chicken (or refrigerate until needed). Next, lie the chicken breasts flat in a large shallow dish. Add the 2 tbsp sunflower oil, the thyme and plenty of salt and pepper and turn the chicken several times until evenly coated. Cover and refrigerate until the barbecue is ready (if grilling indoors, give the chicken at least 15 minutes beforecooking).
When the barbecue is ready, take the chicken out of the fridge and place it directly on the grill. Cook for 5-6 minutes, then turn and repeat – keep going until the chicken is cooked through thoroughly, and any juices run clear. When the chicken is almost ready, put the slices of bread on the grill briefly, just to toast them lightly, then remove the bread and rub each slice with a garlic half. Put a slice of bread on each of four serving plates, then top the bread with a cooked chicken breast and a good spoonful of the tomato topping. Drizzle a little more olive oil over the top of each plate, then serve, with any leftover tomato alongside.