Preheat the oven to 375°F (190°C). Pat the chicken thighs dry.
Combine salt, pepper, garlic powder, onion powder, and dried thyme. Season both sides of the chicken.
Heat one teaspoon of olive oil in an oven-safe skillet over medium-high heat. Sear the chicken for 3–4 minutes on the first side and 2–3 minutes on the second side. Remove and set aside.
Add the remaining olive oil and sliced onions. Cook over medium-low heat for 20–25 minutes, stirring occasionally, until soft and golden brown.
Add minced garlic and cook for 30 seconds.
Stir in beef broth, Worcestershire sauce, balsamic vinegar, Dijon mustard, and dried thyme. Scrape the browned bits from the bottom of the skillet.
Mix cornstarch with cold water and stir into the sauce. Simmer for 1–2 minutes until slightly thickened.
Return the chicken to the skillet and spoon the onion sauce over each piece.
Transfer to the oven and bake for approximately 12–18 minutes, or until the thickest part of each thigh reaches 165°F.
Divide the reduced-fat Swiss cheese evenly over the chicken.
Broil for approximately 1–2 minutes until the cheese melts. Watch carefully to prevent burning.
Finish with fresh parsley and allow to rest for 5 minutes before serving.