Preheat your grill to medium-high heat.
Season the chicken breasts with salt and pepper on both sides.
Place the chicken breasts on the grill and cook for about 6-8 minutes on each side, or until they’re fully cooked and have beautiful grill marks.
While the chicken is grilling, heat olive oil in a pan over medium heat.
Add the thinly sliced onions to the pan and cook, stirring occasionally, until they become golden brown and caramelized, about 20-25 minutes.
Drizzle balsamic vinegar over the caramelized onions and stir to combine. If desired, add a teaspoon of brown sugar for extra sweetness.
Once the chicken is cooked, remove it from the grill and let it rest for a few minutes.
Serve the grilled chicken topped with the luscious caramelized onions.