Arrange a rack in the middle of the oven and heat the oven to 425°F.
Place 1 pound boneless, skinless chicken thighs in a bowl and season with salt and pepper.
If using the sauce, place 4 minced garlic cloves, 2 tablespoons olive oil, 1 tablespoon Dijon mustard, 1 tablespoon balsamic vinegar, and 1 teaspoon packed brown sugar in a medium bowl. Whisk until combined.
Pour the sauce over the chicken and toss to combine.
Place the chicken in a single layer in an 8- or 9-inch baking dish.
Roast until the chicken has an internal temperature of 165°F, about 20 minutes.
Remove the baking dish from the oven, cover with aluminum foil, and let the chicken rest for 10 minutes.
