In a small bowl, mix together the Dijon mustard, olive oil, lemon juice, lemon zest, garlic powder, dried thyme, salt, and pepper to create a marinade.
Pat the chicken thighs dry with paper towels and place them in a large resealable plastic bag or a shallow dish. Pour the marinade over the chicken, ensuring each piece is well coated. Seal the bag or cover the dish and marinate in the refrigerator for at least 30 minutes, or up to 2 hours for more flavor.
While the chicken is marinating, heat the butter in a large skillet over medium heat. Add the sliced onions and cook, stirring occasionally, for about 10 minutes until they begin to soften.
Sprinkle the brown sugar over the onions and continue to cook for another 10-15 minutes, stirring frequently, until the onions are deep golden brown and caramelized. If the onions start to stick, add a splash of chicken broth to deglaze the pan.
Once the onions are caramelized, remove them from the skillet and set aside. In the same skillet, add the marinated chicken thighs, skin side down, and sear for about 5 minutes until the skin is crispy and golden.
Flip the chicken thighs over and add the caramelized onions back to the skillet. Pour the remaining chicken broth around the chicken.
Transfer the skillet to the preheated oven and bake for 25-30 minutes, or until the chicken reaches an internal temperature of 165°F (74°C).
Remove from the oven and let the chicken rest for 5 minutes before serving. Garnish with chopped parsley.
Notes
For a lighter version, use skinless chicken thighs or chicken breasts.
Add vegetables such as bell peppers or zucchini to the skillet for a one-pan meal.