In a medium bowl, whisk together the balsamic vinegar, 2 tablespoons of olive oil, maple syrup, garlic, salt, and pepper.
Place the chicken thighs in a large glass bowl or plastic bag and pour the marinade on top. Marinate in the fridge for 1 to 2 hours.
Preheat the oven to 425°F (220°C). Place the Brussels sprouts and onion on a sheet pan. Drizzle with the remaining 2 tablespoons of olive oil, and season with salt and pepper. Use your hands to mix everything together on the sheet pan.
Remove the chicken from the marinade and place it on the sheet pan, nestling it between the vegetables. Bake for 30 to 40 minutes or until the chicken is cooked through.
While the chicken is baking, pour the remaining marinade into a small saucepan on medium-low heat. Simmer for 8 to 10 minutes or until the sauce is reduced and thickened. Open the oven, brush the glaze on the chicken and continue baking. Repeat this one more time while the chicken is baking. Note: the glaze will continue to thicken while it cools, so it will be much thicker on the second application.
Use an instant read thermometer to ensure the chicken is cooked through to 165°F (74°C), then serve.