1hr 30min · 4 servings Ingredients: • 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 Instructions: • 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 contin