Plunge the tomatoes in boiling water for 1 minute to help make peeling easier. Remove the skin, cut in quarters and remove the seeds. Cut in small cubes, reserve.
Cut the red onion in small dices, add to the tomato dices.
Remove the leaves from the mint.
Crush the garlic cloves, grate and squeeze the lemon. Chop the mint leaves; mix with the olive oil, garlic, fresh pepper, salt, lemon juice and lemon zest. Pour half of the mixture over the chicken and half over the tomatoes and red onion. Let the meat and the tomatoes marinate for at least 30 minutes.
When ready to cook, place the chicken on a hot grill for 1 to 2 minutes per side to get nice markings on the breasts. Remove from the grill and put on a small sheet pan. Pour any remaining marinade over the chicken.
Place in a 350°F oven for 7 to 10 minutes, depending on the thickness of the breast.
Once the chicken is cooked, allow the meat to rest for about five minutes. Mix the chicken's cooking juices with the tomatoes. Slice the chicken and place on a plate, cover with the marinated tomatoes and drizzle remaining tomato marinade over all. Serve with mint-infused couscous.