- 2 tablespoons butter
- 2 pounds yellow onion, thinly sliced
- 1 teaspoon salt
- 2 cups beef broth, divided
- 1/2 cup olive oil
- 3 pounds boneless skinless chicken thighs
- 1 tablespoon Dijon mustard
- 2 tablespoons sherry vinegar
- 2 tablespoons all-purpose flour
- 2 tablespoons minced thyme, plus 8 sprigs
- 2 cups Gruyere, grated
- In a large cast-iron skillet over medium heat, heat butter and add onions in batches. Coat each batch evenly with melted butter and cook until each batch is translucent. Sprinkle with salt and let onions cook without stirring. Reduce heat to medium-low and cook without stirring much for approximately 15 to 20 minutes. If onions begin to stick to the bottom, add beef broth and continue cooking until a golden brown color is reached. Set aside.
- In a large Dutch oven on medium-high heat, add olive oil. Sear chicken thighs on each side, approximately 5 minutes each, and remove. Add remaining beef broth, Dijon and sherry and cook until it bubbles. Add flour and thyme and stir until it thickens. Return chicken back to pot and set aside.
- Preheat the oven to 350 degrees.
- Add the caramelized onions to the Dutch oven and and bake for 25 to 30 minutes until chicken reaches 165 degrees and broth mixture has thickened. Remove from oven and top with Gruyere. Broil for 4 to 5 minutes until cheese melts. Remove from oven and top with thyme sprigs. Serve immediately. Chicken will keep up to 5 days covered in the fridge.