Combine garlic, brown sugar, 2 T. lemon juice, and 3 T. olive oil in a small bowl.
Using a rolling pin, lightly pound chicken breasts between sheets of waxed paper until they are 1/2 inch thick. Place chicken on a plate in a single layer, and coat with the garlic marinade. Season both sides with salt and pepper.
Cook couscous according to package instructions. Fluff with a fork, then add 1 T. lemon juice, 2 T. olive oil, mint, red onion, tomato, and salt and pepper to taste. Set aside.
If grilling outside, cook chicken until browned and just cooked through, about 3 minutes on each side. For indoor preparation, preheat oven to 375. Heat an ovenproof skillet on the stovetop, and cook chicken for 3-4 minutes. Flip, then place pan in oven and bake for 3 more minutes. Serve over couscous.