In a liquid measuring cup or mixing bowl, whisk together the beef broth, vinegar, thyme, garlic pepper and salt. Set aside.
In a large oven-safe skillet, heat the vegetable oil over medium-high heat. Sear chicken breasts on all sides until browned. They will not be cooked the whole way through.
Pour the beef broth mixture over the chicken, heating until at a low simmer.
Top with the caramelized onions and sprinkle with cheese.
Bake for 16-18 minutes or until the chicken registers at 160°F.
Remove and allow to rest for 5-10 minutes (carry-over cooking will bring it up to 165°F).
Before serving, sprinkle liberally with garlic croutons and garnish with fresh thyme or parsley, if desired.
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