Heat your oven to 400°F (200°C). Line your sheet pan with parchment paper or lightly grease it to prevent sticking.
In a medium mixing bowl, combine the balsamic vinegar, olive oil, minced garlic, dried thyme, salt, and black pepper. Whisk vigorously until fully blended.
Add the chicken breasts and halved Brussels sprouts into the bowl with the marinade. Toss everything together until each piece is generously coated.
Spread the coated chicken breasts and Brussels sprouts out evenly on your prepared sheet pan, ensuring enough space to roast rather than steam.
Place the sheet pan in the oven and roast for 20 minutes. Then flip the chicken and stir the Brussels sprouts. Return to the oven and roast for another 10–15 minutes or until the chicken reaches 165°F (74°C) and the Brussels sprouts are tender and browned.
While the chicken roasts, toast the chopped pecans in a dry skillet over medium heat for 3–5 minutes, stirring frequently until fragrant and lightly browned.
Once everything is out of the oven, sprinkle the toasted pecans over the chicken and Brussels sprouts. Serve immediately.
