Cut the tomatoes into wedges, remove the seeds, and dice the flesh into small cubes. Crush the garlic finely and finely chop some of the basil. Mix the tomatoes with some of the basil, half of the garlic, and some of the olive oil; season with pepper, cover, and chill.

Mix the rest of the garlic with the remaining olive oil. Brush the portobellos with it and season with pepper and salt.

Grill the portobellos on a very hot barbecue or bake for 15 minutes in the oven at 392°F until cooked through and lightly browned. Turn them occasionally so they cook evenly.

Fill the portobellos with the tomato mixture and finish with balsamic vinegar and the rest of the basil. Add a little more pepper and salt before serving.