In a medium bowl, whisk together the mayonnaise, mustard, garlic and half of the lemon juice. Set aside 1/3 cup and reserve the rest for the dipping sauce.
Preheat the broiler to high. Leaving the tops of the rosemary sprigs intact, remove some of the bottom leaves to make room for threading. Thread the chicken and tomatoes onto the rosemary sprigs. Brush the kebabs with some of the 1/3 cup mayo-mustard sauce and season with salt and pepper. Arrange on a foil-lined baking sheet and broil about 8 inches from the heat, turning occasionally and basting with the remaining 1/3 cup sauce, until cooked through, about 15 minutes.
Meanwhile, in a medium saucepan, bring 2 cups water to a boil. Stir in the couscous, cover and remove from the heat; let stand for 5 minutes before fluffing with a fork. Stir in the remaining lemon juice, the lemon peel and parsley. Season with salt and pepper. Serve with the kebabs and dipping sauce.