1 tablespoon olive oil
1/2 teaspoon coarse salt
1 tablespoon butter
1/8 teaspoon freshly ground pepper
2 medium yellow onions, sliced
1 1/4 pounds (6 to 8 pieces) sliced chicken cutlets
Coarse salt and freshly ground black pepper
4 slices fontina cheese
1 teaspoon minced thyme
4 split buns or rolls
1 tablespoon Dijon mustard, plus more for serving
Lettuce or arugula for garnish
2 tablespoons extra-virgin olive oil
Heat olive oil and butter in a large sauté pan over medium heat. Add onions, ½ teaspoon salt, and a pinch of pepper; stir well. Cover pan for 5 minutes to allow onions to release liquid. Reduce heat to medium low and cook, uncovered, for 40 minutes.
Cook onions, stirring occasionally and scraping any browned bits from the bottom of the pan. (If onions begin to brown too fast, reduce heat.) Stir in thyme (1/2 teaspoon) and remove from heat.
In a mixing bowl, combine mustard, olive oil, thyme (1/2 teaspoon), salt, and pepper; mix well. Season chicken cutlets with salt and pepper and toss them in mustard marinade; marinate for 15 minutes.
Heat oven to 250 degrees. Heat a grill pan over medium-high. Add chicken pieces to grill pan and cook, turning once, until they have grill marks and are no longer pink in the middle, about 6 minutes total.
Divide cooked chicken cutlets into 4 portions and place on a rimmed baking sheet. Lay a slice of cheese over each portion and bake until cheese melts.
Serve chicken on buns topped with caramelized onions, arugula, and mustard.