Preheat oven to 350°F and prepare a large baking sheet by greasing with cooking spray or lining with parchment paper.

In a small bowl, whisk together the balsamic vinegar, dijon mustard, maple syrup (or brown sugar), lemon juice if using, and 1 tablespoon of olive oil.

Pat chicken thighs dry with paper towels and place them in a large bowl. Pour the balsamic mixture over the chicken and toss to coat evenly. Let the chicken marinate for at least 10–15 minutes while you prep the vegetables.

Wash and slice the brussels sprouts in half. Cube the sweet potatoes into small, even pieces. If using red onion, slice into wedges or thick strips.

Place brussels sprouts, sweet potatoes, and onion onto the prepared baking sheet. Drizzle with the remaining tablespoon of olive oil and a little extra balsamic glaze if desired. Season with garlic & herb seasoning, salt, and black pepper. Toss to coat.

Add the marinated chicken thighs to the baking sheet, spreading everything out into a single layer. Pour any leftover marinade over the top of the chicken and veggies.

Bake for 30 minutes or until the internal temperature of the chicken reaches 165°F. Use a meat thermometer to check for doneness.

For extra color and caramelization, broil on medium heat for the last 2–3 minutes, watching closely to avoid burning.

Remove from oven and let rest for 5 minutes before serving. Store leftovers in an airtight container in the fridge for up to 3 days.