It's so straightforward to put everything under the broiler at once and remove each component as finished—when the bread is toasted, the romaine is wilted, the chicken is cooked, and the tomato is juicy.
Stir together mayonnaise, mustard, cheese, and zest in a small bowl, then coat top of cutlet with about 2 tablespoons sauce, reserving remaining sauce. Transfer chicken to baking sheet.
04Brush cut sides of tomato with oil and season with salt and pepper, then sprinkle with herbes de Provence. Transfer to baking sheet.
05Toss romaine with 1 tablespoon oil, lemon juice, and 1/8 teaspoon each of salt and pepper in a bowl until coated. Transfer to baking sheet.
06Broil 3 to 4 inches from heat until bread is golden-brown, about 2 minutes. Turn bread and continue broiling until browned, about 2 minutes. Remove toast from broiler and continue broiling until romaine is wilted and well browned in spots, chicken is just cooked through, and tomato is lightly browned, 1 to 4 minutes more (remove romaine, chicken, and tomato as done).
07Rub toast with cut side of garlic (reserve other 1/2 clove for another use). Top garlic toast with chicken, lettuce, and tomato. Serve with remaining sauce.