Season chicken: Pat chicken cutlets dry with paper towels. Season both sides with garlic powder, onion powder, dried herbs, salt, and black pepper.

Sear in skillet: Heat olive oil in a large 12-inch oven-safe skillet (preferably cast-iron) over medium-high heat. Add chicken cutlets and sear undisturbed for 4 to 5 minutes per side until golden brown and cooked to 165°F (74°C). Transfer chicken to a clean plate.

Caramelize onions: Reduce heat to medium-low. Add butter to the same skillet. Add sliced onions and 1/4 teaspoon salt. Cook slowly, stirring frequently, for 20 to 25 minutes until the onions turn rich, jammy, and deeply golden-brown. If the pan browns too fast, add a splash of broth.

Build savory sauce: Add minced garlic, flour, and fresh thyme sprigs; cook for 1 minute until fragrant. Pour in beef bone broth, Worcestershire sauce, and balsamic vinegar. Scrape up any browned bits stuck to the bottom of the pan. Simmer for 3 to 4 minutes until the sauce thickens into a rich, velvety onion gravy.

Assemble & top: Discard thyme stems. Nest seared chicken cutlets back into the pan, spooning rich caramelized onions and gravy generously over each piece. Drape each cutlet with a slice of provolone cheese, then heap shredded Gruyere evenly over top.

Broil until bubbly: Transfer skillet to the oven and broil on high for 3 to 4 minutes until the cheese is bubbling, melted, and delightfully blistered with golden-brown spots.

Garnish & serve: Remove carefully from oven with heatproof mitts. Garnish with fresh thyme leaves and cracked black pepper. Serve immediately with warm crusty French baguette to sop up the pan gravy!