
In a large cast-iron skillet or Dutch oven pot, melt 2 tablespoons of butter over medium heat. Add the sliced onions and a pinch of salt. Cook, stirring occasionally, for about 25-35 minutes until the onions are deep golden brown and caramelized. Add 2 ounces of the beef broth to keep the onions from sticking to the pan.
While the onions cook, season the chicken thighs with salt, pepper, and the Herbs de Provence. Remove the onions from the pan and set aside. Melt the remaining 2 tablespoons of butter in the same pot used for the onions and sear the chicken thighs over medium-high heat until golden brown on both sides (about 5 minutes per side).
Add the minced garlic, fresh thyme sprigs, and the caramelized onions back into the pan with the chicken. Pour in the beef broth. Lower the heat and simmer for 10–12 minutes until the liquid has reduced by half and the chicken is cooked through.
Lightly toast your bread slices on a baking sheet under a broiler for just a minute. Leave the toasted bread on the baking sheet for assembly.
Remove the chicken from the pan and slice it into strips. Place the toasted bread on a baking sheet. Lay the sliced chicken onto the bread. Top generously with the caramelized onion and broth reduction. Smother with the Gruyere cheese.
Place the baking sheet under the broiler for 2 minutes, or until the cheese is bubbly and slightly browned. Serve immediately while hot.