Preheat your oven to 400°F and line a large sheet pan with parchment paper for easy cleanup.
Peel the carrots and slice them into 1/2-inch thick rounds, and slice the red onion into 1/2-inch thick wedges.
In a small mixing bowl, whisk together the olive oil, maple syrup, Dijon mustard, balsamic vinegar, minced garlic, sea salt, and black pepper until well combined.
Place the chicken breasts, sliced carrots, and onion wedges onto the prepared sheet pan.
Pour the maple-Dijon glaze over the chicken and vegetables, using your hands or a spatula to toss everything until thoroughly and evenly coated.
Spread the ingredients out into a single layer on the pan, ensuring the chicken pieces are not crowded so they brown properly.
Roast in the center of the oven for 20 to 25 minutes, or until the chicken reaches an internal temperature of 165°F and the carrots are fork-tender.
Remove the pan from the oven and let the chicken rest for 5 minutes before garnishing with fresh thyme leaves and serving.