In a small bowl, whisk together honey, Dijon mustard, olive oil, salt, and pepper until well combined. This will be your glaze and marinade for the chicken.
Flatten the chicken breasts gently to even thickness for uniform cooking. If you have a meat mallet, a light ply works. Coat the chicken with a thin layer of the honey-mustard mixture and let it marinate for 10 minutes while you heat the grill or grill pan.
Preheat your grill to medium-high heat. Oil the grates lightly or brush the chicken with a touch more oil to prevent sticking. Grill the chicken about 5–6 minutes per side, or until the internal temperature reaches 165°F (74°C). During the last minute of cooking, brush with a little more honey-mustard glaze to build flavor.
While the chicken rests for a few minutes, toast the bread or buns until golden. If you like mayo, mix the remaining teaspoon of mayonnaise with a touch of honey and Dijon for a quick creamy spread.
Assemble each sandwich by layering greens on the bottom bun, followed by a sliced grilled chicken breast. Top with tomato slices, cheese if using, and a drizzle of any extra glaze. Cap with the top bun.
Serve immediately with a light side like fruit, carrot sticks, or a simple pickle spear. The result should be a balanced bite—sweet, tangy, and a little smoky from the grill.